Collecting for Photography is easy, I'd get paid when I delivered the DVD with all the shots burned on it. But that's the trouble with the Digital Age, people are reluctant to pay a pro when the "easy" button on cameras today delivers excellent technical results in most cases. By the same token, today's photo software makes darkroom tricks available to everyone. More people are willing to pay premium rates for expert tree work than are willing to pay for expert and artistic photo and video coverage of their wedding.
I've got thousands of dollars worth of studio grade tripods and a wide assortment of specialized heads for them that have been rendered pretty much obsolete by Nikon's Nikkor "VR" lenses. Even point and shot cameras and phones have movement sensors to steady your hand held shots. Tripods are nearly obsolete, I still like 'um tho, just bought two more, a carbon fiber Manfrotto and it's aluminum twin.
I've never had a late payment or collection issue with tree work. I do the work, they pay me, then we leave. I make the terms quite clear in the initial evaluation. If they have trouble paying at that time, no problem, just make up the guest room and tell us what's for dinner.