[fix] Instance usage of get_payment_method_info in billing service

[nwithan8]

Description

As pointed out in #295 , a helper method in our billing service was static rather than instance-based, causing issues when calling it.

This PR makes the method a private instance helper method (not for end-user consumption), and updates the associated unit tests.

Closes #295

Testing

• Unit test updated, no cassettes need to be rerecorded (test was already mock-only)

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[Justintime50]

"How was this not caught"

Ah yes, mocking...

[jchen293]

Need a changelog