Our payment obligation?

Like most Americans, we owe some money. We are slowly paying it back, and making some progress. What we try to do is keep current on things and periodically pay things off in one big lump sum, since that gets us around the issues of murderous interest. Even bills without interest, I do a similiar thing, paying small bits of the amount owed, just to show them that we know they are there and we intend to pay them off.

In the case of most of these owed bills, I'll pay them $X amount every time they send me a notice. I do that because the notices come about once per month, and they provide an envelop. We've got a ton of timeless stamps, and I figure I might as well use them up while we are transitioning back to completely electronic banking again.

So imagine my surprise when one debtor, in their letter to me, state that my due payment of $X is due on October 15th 'per my obligation to pay $X per month'?

This is really funny. I owe these people money and that is not in doubt. What is in doubt is that I ever entered an obligation to pay a specific amount each month. I never did that with anyone and yet these people claim that I have agreed to an obligation.

Of course I'll keep paying because its good to clear this thing out, but I find it amusing that they noticed my ongoing constant payments and changed their nastygrams to reflect it.