Today, I'm sending them a letter by certified mail, informing them that I expect them to reduce the interest rate to what it was, and remove any interest or fees if it takes longer than the early July due date for the next payment, and to close the account (because I don't do business with people who try to rip me off).

I also informed them that I have no intention of making any more payments on this account until they do these things.

They were also instructed to contact me only by mail, not by email or phone, to document what they have to say. Otherwise, I fear there will be "misunderstandings."

We'll see if they can follow simple instructions.

(As a side note, this is being sent to the payment address, which is almost certainly the wrong address for anything but payments, but it is literally the only mailing address I have on any correspondence from them, so it's the address they choose for me to send all correspondence to.)