If the ex wouldn't take a check and call it good... you need/want proof so I would make a call to the attorney that handles things and ask them to deliver payment and get a receipt.I spoke with my ex regarding the matter and she wouldn't take a check from me
If you are to the point that your employer is withholding CS, you are probably better off keeping everything documented and through an attorney. (even if on speaking terms with the ex)
Good luck!!