Medical collection agency REMEX - PIF without a letter to guarantee deletion?

[tsally862]

I contacted REMEX today and I was told that if I make a full payment of $856 before the end of the month, that they would delete the collection from my credit report. However, when I asked for them to send me a letter stating that they would in fact make such deletion, I was told they cannot provide me with one. They said I will receive a letter from them stating I have paid in full, and to then call them back so that they can remove the collection from my report. Without anything in writing, I am very reluctant to send out a money order with no guarantee that the collection will be deleted. However, I am trying to buy a car soon and going through the HIPPA process will just take too long right now. Any recommendations? Also, what kind of increase in the credit score could I expect with this one deletion? I will have 3 collections left but I guess 3 is better than 4, right? Thank you.