I send the invoice when I have finished the map. If I have measured for them before, and they have paid in a timely manner, I will send the cert and map at the same time. If I have not measured for them before, or I know they are slow-pays, I let them know I will send the map and cert to them when I receive payment. I tell them up-front that cert and map won't be sent until paid, so there is not an issue of unexpected timing.
If I travel out of town, there are travel expenses. Since it is normally for a marathon, there is also lots of work involved, so I normally receive half the fee before I make the trip, with the balance due before I leave town. I have not had anyone complain about this method. Since I draw most of the map before I measure, it only takes me a day, or so, to send the map after I get home.