I've had this happen. From a fairly reputable guy too. I wont name names because I'm over it...and I was really happy with the quality of the piece.
Commissioned a piece for ECCC 2014, paid up front. Artist had to cancel his appearance...no problem. I said I'd pay for shipping when he had a chance to complete it. Again, no problem. Some months roll by, piece isn't completed, but I knew that would be the case, so I just set up pick-up for NYCC. A friend of a friend at the time (now a good pal....you know who you are) offered to pick it up for me and ship it....still all good. I wasn't even worried about the fact that I paid shipping for a piece that was getting picked up...just a sign of good faith/appreciation. When my friend arrives to pick-up the piece at NYCC, the artists rep tells him that I still owe $50 on the piece (I had paid in full in advance). Through email, my friend tells me the predicament and that I still owe the rep money. I tell him that I paid in full, well in advance of ECCC (the show it was originally commissioned for), the rep tells him that prices increased a few months ago (after ECCC). I tell him to tell the rep that I paid ECCC prices because I was picking up for ECCC...it was the artist who had to back out. The rep tells him to tell me that I still need to pay the difference because of the price increase (despite the fact that the prices were the prices when I commissioned it). I reminded him that I had already paid shipping on a piece that was getting picked up, so I subtracted the shipping from the $50 difference and made paypal payment of like 17 dollars. I would've argued this further if I was picking up in person, but since this was a friend of a friend at the time, I didn't want to burden him with this virtual argument.
Long Story Short - I paid the March price for a pick-up in April. The artist was unable to complete my piece by then, prices increased in like July or August, and this rep (in all his infinite wisdom) figured I needed to pay the difference....jackass.
Love the piece though