Tactless letters

I received multiple nasty letters, even after I paid them.
Stating, and I quote, " I don't understand why you've not paid this bill... You asked for credit and we gave it to you. You agreed to pay us.... Since you have failed to respond to repeated requests for payment, I can only assume that you do not intend to honour your commitment ..." . This is a huge company. You would think they would have classier people to write their letters. You would also think they would have their stuff together.