Unexpected/ duplicate charges for dormant US marketplace account

We operate on Amazon UK without issue, however many years ago (maybe a decade or more) we also operated on Amazon US – back then Amazon US was treated as a completely separate entity with a separate login, with no connection to the UK side of things.

After some changes in Royal Mail regulations we were no longer able to send most of our products (Colognes and Aftershaves) to the US so decided to cease operation on Amazon US completely. However, suddenly since February this year Amazon US has started charging us around $20USD a month again and we’re not sure why. Surely we don’t have to pay a separate monthly fee for the US marketplace (especially considering we haven’t used it in ages)? But then we’re also wondering why they haven’t been doing this for all the other years that the account has been dormant.

Ideally we’d just want to close the US account outright, however it now looks like the accounts have been merged and it’s been changed to a country/ marketplace region selector. Is there a way for us to close the US arm of the account completely without also closing the UK account? We just can’t get a straight answer out of Amazon support at all as they just keep reminding us that the card on the US account has expired (which we already know – and is deliberate because we no longer wish to use it), however they have begun forcibly taking the USD charges from our attached bank account now instead.

Any advice on this would be greatly appreciated as they are currently taking two separate monthly charges from us (UK/ GBP and US/ USD) when we don’t even want or need the latter to remain active.