How trustworthy are collection agencies when responding to a Settlement Letter?

I've read some of the posts in here about problems others have had with collections on accounts that should have been settled. If I send in the settlement amount, can I generally expect them to settle the account as they promise? Do they have the authority to do that?

Also, if I have a friend who is willing to pay the settlement amount for me, can he just write them a check or does the check have to be in my name?

I'm a little leary dealing with these people. I'd rather not let them know that they've found me but I would like to get it behind me too. I have had perfect credit for about 7 years but at least one old debt that I defaulted on when I was young and irresposible still nags me. Trying to determine if I should pay it off and put it behind me or not. I worry if this letter can be trusted.

Any help or pointers to more info is much appreciated.

Shirtless1