Gerry,
Prepayment on S.O.P's can be sticky as pointed out for customer pay, so we simply recorded credit card info to cover our butts, particularly on Keys, and painted mirror covers etc. We then had the option to simply run it though and mail the stuff if the shipping wasn't a big factor. But generally commitment by card was good enough to move the part.
Most effective in moving the warranty stuff ordered by service was implementing a system that held the advisors "ACCOUNTABLE!!" I set up a job stack report that printed out S.O.P.'s by advisor in descending order of date, complete with contact#s, part#s & descriptions etc. A copy was provided to the advisors and service manage along with a similar listing of B.O. components every morning. Aged stuff was highlighted as critical. Each advisor was required to date and time each SOP line and submit by days end. It was very clear in a short time who was not making calls! And the problem was immediately remedied.
As for policy debates with repect to return charges for SOP's, I would tread lightly. I can certain recall a good number of times cars were held over cause a part got cross shipped, was damaged in stock, was missing on the shelf, showed up too late, missed the VOR, etc and cost service both stress and a rental. I advocate tracking trends. Examine for careless techs, or advisors. If a "Real" excessive problem exists, take action, if not nickle and dimeing is probably not worth it.