I used to work for Collect Corp. They have been taking a bloodbath on market share with Amex for not performing.
1. They take post dated checks and can deposit checks into their account because they have a client trust account. They are legal agents for their clients, like most agencies.
If you are being threatened with a lawsuit from Collect Corp. and they will not provide you anything in writing, simply call Amex and ask if they have authorized suit. If they have not file a complaint with Amex, the BBB, you state attorney general, and the FTC.
I wouldn't deal with Collect Corp directly if they are threatening you and using abusive tactics to collect the debt.