Having read this post, plenty of points of view has been put across..
I've painted shells on and off for well over 20+ years, I have always asked for payment for any painting work carried out AFTER the work has been completed, and confirmation pictures sent. Only time when I charge customers for paintwork is done only if specifically asked for this to be done by the customer
I found this to work out quite well, and so have the guys I have painted for (must admit sometimes the wait can be a bit long! but all good things come to those who wait
)