how do i pay for the monthly apple plan?

The Monthly plan is tied to the Apple ID and would use its payment method regardless of where it was bought. There's to my knowledge no other way to get the monthly payment for Apple Care.

The one time purchase Apple Care can be paid with whatever payment method is accepted wherever it is bought.

Its possible what was obtained was a one time purchase Apple Care but financed for monthly payments through a different payment method such as the Best Buy card. In that case, its not technically the Monthly Apple Care. Its the single purchase Apple care, financed into monthly payments outside Apple's process.

If it does not Appear as a subscription to Manage in the Apple ID, then as the support document suggests it would be best to contact Apple Support.

Note that this is all mostly speculation on my part based on what we know about the Monthly based Apple Care+ and what we discussed in your thread in the WC.

If you buy Monthly Apple Care+ it would need to be activated with your Apple ID is what I'm saying. 3rd party authorized retailers have access to Apple's systems and activate the regular warranty as well as Apple Care at the moment of purchase.

So if the monthly Apple based Apple Care+ option was bought at the same time you buy the iPad, it gets activated in the system on the Apple ID you provide to them.

However, if the monthly payments are done outside the Apple system, then its like buying a phone and financing it for 12 months or 18 months, interest free. You are technically buying the one time Apple Care+ , but spreading the payments over a longer period of time outside Apple's own system for it. This would not activate the Monthly Apple Care on Apple systems, and as such would not be manageable as a subscription.

Key point here, is if it had been activated as a monthly Apple Care+ subscription it cannot have been activated outside of the Apple ID and using its payment method.

The fact its using the Best Buy card, leads me to believe its not the actual Monthly Apple Care+ but the single purchase Apple Care+ spread into monthly payments on the Best Buy Card. In which case as you mentioned originally, they would need to go back to Best Buy to change the associated payment method with it.

Hope that makes sense.

If it is on your Best Buy credit card, you need to approach Best Buy for help in changing it to another card (and I have absolutely no idea about their credit card policies/practices).

The monthly Apple Care plan is paid using whatever payment method you have on file on your Apple ID for iTunes and App Store, including any iTunes Gift card credit balance.

See here: How to manage your AppleCare plan - Apple Support

Does that also apply for an AC plan purchased at a retailer (along with a device)? If it is not purchased directly from Apple, the payment method would be whatever card was used at the retailer (in this case a Best Buy card)?

Ahhh, my bad - I did not realize that the monthly was treated like a "subscription". But I cannot wrap my brain around this: if I walk into a third party retailer, buy an iPad and monthly Applecare+, how do they differentiate the payment methods? They are not necessarily hooked into the Apple world and may not know that they need to bill the AC to the card tied to my Apple ID? They may have to split the billing between two cards?

Thanks for the detailed explanation!

3rd party authorized retailers have access to Apple's systems and activate the regular warranty as well as Apple Care at the moment of purchase.

That is the point I was not clear on: the last time I bought something from an authorized reseller (and that was quite some time ago), they did NOT have access to Apple's system. I had to activate both the device warranty and Applecare (which I purchased separately elsewhere) myself.

As far as I know, that's no longer the case. They activate at the time of purchase now.

At least it worked that way when I bought my Apple TV and Apple TV 4K last time from authorized resellers.

Warranty was active when I left the store.