Zelle is quick and easy, just be sure the transfer is going to whom you think it's going. As their site says, Zelle is designed to facilitate transfers between friends, or at least individuals personally known. It is not intended for commercial payments. I think personal checks are okay, assuming proper identification and sufficient time for collection between banks, usually about a week. Counterfeit money orders and official checks are very prevalent, so if it's issued by a bank, it should be verified. USPS, I believe has an online MO verification available.