I've been wanting to add fiction commissions for a while, and this is honestly one of my big sticking points.  I've always been fairly good at sticking to a set wordcount, but sometimes things change.  A buyer wants more scenes or wants to cut the project in half before you're done with it, and that's hard to adjust.

My physical item commissions, on the other hand, NEED to be paid up front.  My material costs can be stupidly high, and I can't afford that out of pocket most of the time.  The buyer often winds up paying for the materials because I don't have them on hand, or run out halfway through.