Paid newsletters fix warnings

[artpi]

More context in p1674830085351189-slack-CBG1CP4EN

Fixes the call where we would read the global $post without any protection against it being null

Proposed changes:

use get_the_ID() instead of the $post global

Does this pull request change what data or activity we track or use?

No

Testing instructions:

Same as #28170

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[github-actions]

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