diff --git a/packages/capabilities/src/types.ts b/packages/capabilities/src/types.ts
index f1eb89800..37c6c05e3 100644
--- a/packages/capabilities/src/types.ts
+++ b/packages/capabilities/src/types.ts
@@ -634,6 +634,9 @@ export type PlanSet = InferInvokedCapability<typeof PlanCaps.set>
 
 export type PlanSetSuccess = Unit
 
+/**
+ * @deprecate currently unused - used to be part of PlanSetFailure but we switched to CustomerNotFound
+ */
 export interface AccountNotFound extends Ucanto.Failure {
   name: 'AccountNotFound'
 }
@@ -642,7 +645,7 @@ export interface InvalidPlanName extends Ucanto.Failure {
   name: 'InvalidPlanName'
 }
 
-export type PlanSetFailure = AccountNotFound
+export type PlanSetFailure = CustomerNotFound
 
 // Top
 export type Top = InferInvokedCapability<typeof top>
diff --git a/packages/upload-api/src/plan.js b/packages/upload-api/src/plan.js
index 0f22c7b3c..47a80a224 100644
--- a/packages/upload-api/src/plan.js
+++ b/packages/upload-api/src/plan.js
@@ -1,6 +1,50 @@
 import * as Types from './types.js'
 import * as Get from './plan/get.js'
 
+import { Failure } from '@ucanto/server'
+
+export class CustomerNotFound extends Failure {
+  /**
+   * @param {import('./types.js').AccountDID} accountDID
+   */
+  constructor(accountDID) {
+    super()
+    this.accountDID = accountDID
+  }
+
+  /**
+   * @type {'CustomerNotFound'}
+   */
+  get name() {
+    return 'CustomerNotFound'
+  }
+
+  describe() {
+    return `${this.accountDID} not found`
+  }
+}
+
+export class CustomerExists extends Failure {
+  /**
+   * @param {import('./types.js').AccountDID} accountDID
+   */
+  constructor(accountDID) {
+    super()
+    this.accountDID = accountDID
+  }
+
+  /**
+   * @type {'CustomerExists'}
+   */
+  get name() {
+    return 'CustomerExists'
+  }
+
+  describe() {
+    return `${this.accountDID} already exists`
+  }
+}
+
 /**
  * @param {Types.PlanServiceContext} context
  */
diff --git a/packages/upload-api/src/types/plans.ts b/packages/upload-api/src/types/plans.ts
index a94a204ae..6bfc06a12 100644
--- a/packages/upload-api/src/types/plans.ts
+++ b/packages/upload-api/src/types/plans.ts
@@ -3,12 +3,35 @@ import { AccountDID, DID, PlanGetFailure, PlanGetSuccess, PlanSetFailure, PlanSe
 
 export type PlanID = DID
 
+export interface CustomerExists extends Ucanto.Failure {
+  name: 'CustomerExists'
+}
+
+type PlanInitializeFailure = CustomerExists
+
 /**
  * Stores subscription plan information.
  */
 export interface PlansStorage {
   /**
-   * Get plan information for an account
+   * Initialize a customer in our system, tracking the external billing
+   * system ID and the plan they have chosen.
+   * 
+   * Designed to be use from, eg, a webhook handler for an account creation event
+   * in a third party billing system.
+   * 
+   * @param account account DID
+   * @param billingID ID used by billing system to track this account
+   * @param plan the ID of the initial plan
+   */
+  initialize: (
+    account: AccountDID,
+    billingID: string,
+    plan: PlanID
+  ) => Promise<Ucanto.Result<Ucanto.Unit, PlanInitializeFailure>>
+
+  /**
+   * Get plan information for a customer
    *
    * @param account account DID
    */
@@ -17,13 +40,13 @@ export interface PlansStorage {
   ) => Promise<Ucanto.Result<PlanGetSuccess, PlanGetFailure>>
 
   /**
-   * Set an account's plan. Update our systems and any third party billing systems.
+   * Set a customer's plan. Update our systems and any third party billing systems.
    *
    * @param account account DID
-   * @param plan the DID of the new plan
+   * @param plan the ID of the new plan
    */
   set: (
     account: AccountDID,
-    plan: DID
+    plan: PlanID
   ) => Promise<Ucanto.Result<PlanSetSuccess, PlanSetFailure>>
 }
diff --git a/packages/upload-api/test/handlers/plan.js b/packages/upload-api/test/handlers/plan.js
index 9c8e08855..a718fb4a5 100644
--- a/packages/upload-api/test/handlers/plan.js
+++ b/packages/upload-api/test/handlers/plan.js
@@ -11,8 +11,9 @@ import { Absentee } from '@ucanto/principal'
 export const test = {
   'an account can get plan information': async (assert, context) => {
     const account = 'did:mailto:example.com:alice'
+    const billingID = 'stripe:abc123'
     const product = 'did:web:test.web3.storage'
-    await context.plansStorage.set(account, product)
+    await context.plansStorage.initialize(account, billingID, product)
     const connection = connect({
       id: context.id,
       channel: createServer(context),
diff --git a/packages/upload-api/test/storage/plans-storage-tests.js b/packages/upload-api/test/storage/plans-storage-tests.js
index d07ed0f06..00dda9661 100644
--- a/packages/upload-api/test/storage/plans-storage-tests.js
+++ b/packages/upload-api/test/storage/plans-storage-tests.js
@@ -1,19 +1,50 @@
 import * as API from '../../src/types.js'
 
+const account = 'did:mailto:example.com:alice'
+const billingID = 'stripe:abc123'
+const product = 'did:web:free.web3.storage'
+
 /**
  * @type {API.Tests}
  */
 export const test = {
-  'should persist plans': async (assert, context) => {
+  'can initialize a customer': async (assert, context) => {
+    const storage = context.plansStorage
+
+    const initializeResult = await storage.initialize(account, billingID, product)
+
+    assert.ok(initializeResult.ok)
+
+    const getResult = await storage.get(account)
+    assert.equal(getResult.ok?.product, product)
+  },
+
+  'should not allow plans to be updated for uninitialized customers': async (assert, context) => {
     const storage = context.plansStorage
 
-    const account = 'did:mailto:example.com:alice'
-    const product = 'did:web:free.web3.storage'
     const setResult = await storage.set(account, product)
 
-    assert.ok(setResult.ok)
+    assert.ok(setResult.error)
+    assert.equal(setResult.error?.name, 'CustomerNotFound')
+  },
+
+  'should allow plans to be updated for initialized customers': async (assert, context) => {
+    const storage = context.plansStorage
+
+    const initializeResult = await storage.initialize(account, billingID, product)
+
+    assert.ok(initializeResult.ok)
 
     const getResult = await storage.get(account)
     assert.equal(getResult.ok?.product, product)
+
+    const newProduct = 'did:web:expensive.web3.storage'
+
+    const setResult = await storage.set(account, newProduct)
+
+    assert.ok(setResult.ok)
+
+    const newGetResult = await storage.get(account)
+    assert.equal(newGetResult.ok?.product, newProduct)
   },
 }
diff --git a/packages/upload-api/test/storage/plans-storage.js b/packages/upload-api/test/storage/plans-storage.js
index 041b3248c..6a892ef88 100644
--- a/packages/upload-api/test/storage/plans-storage.js
+++ b/packages/upload-api/test/storage/plans-storage.js
@@ -1,3 +1,4 @@
+import { CustomerNotFound, CustomerExists } from '../../src/plan.js'
 import * as Types from '../../src/types.js'
 
 /**
@@ -6,14 +7,28 @@ import * as Types from '../../src/types.js'
 export class PlansStorage {
   constructor() {
     /**
-     * @type {Record<Types.DID, {product: Types.DID, updatedAt: string}>}
+     * @type {Record<Types.DID, {product: Types.DID, billingID: string, updatedAt: string}>}
      */
     this.plans = {}
   }
 
+  /**
+   * 
+   * @param {Types.AccountDID} account 
+   * @param {string} billingID 
+   * @param {Types.DID} product 
+   */
+  async initialize(account, billingID, product) {
+    if (this.plans[account]) {
+      return { error: new CustomerExists(account) }
+    }
+    this.plans[account] = { product, billingID, updatedAt: new Date().toISOString() }
+    return { ok: {} }
+  }
+
   /**
    *
-   * @param {Types.DID} account
+   * @param {Types.AccountDID} account
    * @returns
    */
   async get(account) {
@@ -32,15 +47,16 @@ export class PlansStorage {
 
   /**
    *
-   * @param {Types.DID} account
+   * @param {Types.AccountDID} account
    * @param {Types.DID} product
    * @returns
    */
   async set(account, product) {
-    this.plans[account] = {
-      product,
-      updatedAt: new Date().toISOString(),
+    if (!this.plans[account]) {
+      return { error: new CustomerNotFound(account) }
     }
+    this.plans[account].product = product
+    this.plans[account].updatedAt = new Date().toISOString()
     return { ok: {} }
   }
 }