Hi Kay1345,
welcome to the Apple (user-to-user) Community.
re: "Billing issue": "expected to pay for an inactive subscription I cancelled weeks ago"
Seems free trials need to be cancelled at least 24 hours before they expire, if you do not want to be charged.
or
If the subscription is one of Apple's own products, their method is to pay what's owing and claim a refund.
If money has been deducted (not pending)
Request a refund for apps or content that you bought from Apple - Apple Support
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Re: Any advice other than pay it?
Of course it is entirely up to you; also you have much more info. than we do about who is responsible.
There are various avenues below, which you could try ... you may or may not want to pursue the matter.
If the subscription is from an app developer, or from a third-party (non-Apple Company.
The best source of help for a product not made by Apple (a third-party product)
is the company or person who made it—its vendor or developer:
How to: Contact a third-party vendor - Apple Support
or
How to contact an app developer - Apple Support
For a subscription to an app from the App Store:
You can: Find the URL for an app in the App Store to report a concern - Apple Support
If needed:
You might want to reach out to Apple's Billing and Subscriptions support (eg: online chat / phone / email etc.)
There is a "Get support-start now" link at the bottom of their website (link below):
Subscriptions and Billing - Official Apple Support
All the best :-)