If you have a bank account, you should be able to go to the bank and ask for a blank check even if you do not regularly order checks. Your bank will track whether he cashes the check.

Whether you pay by check or get a cashier's check, make a copy of it, keep whatever receipt your bank gives you. Any good bank will allow you to call to verify the cashier's check was cashed.

Make a copy of everything you send. A copy of the check. A copy of whatever the bank gives you. Take a picture of the certified mail you give the post office and keep the receipt.

Once you receive confirmation the delivery was accepted (which you can do online), call the office and verify they received your payment. Ask for the name of the person you are speaking with. If you know the email address of the landlord, follow up with an email "I just spoke with Vicki who confirmed you received the rent check for 123 Address."

If you receive an eviction notice, you have everything to prove you sent it, and you can go back to the bank, cancel the check and offer to wire the money.