You bring up an interesting point. So many artists I've commissioned require full payment upfront that I assume it's the case for any new artist I commission. I understand why they do this, because the anonymity of the exchange means commissioners could easily burn them if payment were required after the completed art is provided. However, the reverse holds true. Artists paid up front can easily burn the commissioner by not providing the art. This has indeed happened to me on several occasions. While I imagine it's rarer than commissioners burning artists, since the artists have somewhat less anonymity, if you compare this practice to pretty much any other form of business, it's actually quite unusual. How often are customers charged before a service is rendered or a good provided? And even in those scenarios in which the charge comes first, there is generally accountability, in the form of tracking numbers, formal customer support, etc.

I'm not sure of the best solution. Some artists will complete the commission and make a low-res version available to the commissioner as proof of completion, and only then is the payment required. But artists could still be cheated out of time/energy (and ultimately money) if the customer burns them. Maybe a half upfront, half upon completion policy? Hard to say.