debt settlement with BCR without any papers

After some time, I am now able to negotiate with these ppl. I have agreed to something on the phone with one of the reps but I do not feel comfortable providing account info. I've been asking for a written statement were terms are specified (I want to pay once for the amount we discussed) but I'm afraid that since they don't want to send me anything in written only after I pay that they would try to get more money from the acct. I said I could send a certified check and they refuse saying that if we do it that way then it'd be considered only a payment of the full amount I owe.
I'm thinking about opening an account for that amount, give them the info and immediately close it but I'm also scared that could hurt me if they want more money... are these people to trust?? IS IT EVEN LEGAL TO NOT PROVIDE A PAPER STATING THE TERMS?? I have asked them for this many many ways several times and I'm not sure if they are breaking any laws... PLEASE HELP