Learned this lesson early on while transporting.
A few of my first transports, I agreed to payment upon delivery. After getting a couple of hot checks or promise of payment being put in the mail that never arrived, I now will only transport with half payment prior to loading and the rest in cash before I'll unload the cargo.
Has only happened once but, I was pulling out of a driveway with the load due to non-payment of the second half and all of a sudden the customer remembered where he had the rest of his cash.
Man, that is so sleazy. I assume you make them sign a contract ?