You can have a cashier's check from your bank and letter sent by certified mail to your ex-husband. Make a copy of the letter and check you send him. Sending it by certified mail will give you proof as to whether or not he received the check. A cashier's check from the bank makes a record of who you are giving that money to. If he cashes that check there's more proof that he received it.
The check should be made out directly to your ex, his attorney's name shouldn't be on the check at all. The only reason a check should have an attorney's name on it is if the attorney took a case on a contingency fee, since this was a divorce my guess is your ex paid up front. Also, you would be informed to send it to his attorney with his attorney's name on it.
You could take an extra step of caution and have a cashier's check with your ex husband's name only on it sent by certified mail to his attorney.

