Your relationship is almost certainly already f%cked up.
Why?
Because if you do not fix it, chances are good that they will again use the wrong way to pay, and you won't get your money.
So chances are you'll end up fixing this via the legal department.
We had it once the other way around, when our ISP was swallowed by a bigger one, the new accounting Dept was incapable of assigning payments correctly. (Yes we have a complicated address.)