this doesn't address the posting about how to charge for the microwave bowls. Just wanted to comment on the ones that I've made ...
Last year I purchased the Warm & Zap on a bolt from Amazon, best price after comparing Joann's etc. As I ran low I used 1 of the warm / zap and one warm & natural. This year I only used warm & natural. When I gave them as gifts I included directions NOT to use more than 2 minutes cuz they might catch fire.
No one has told me they've had them burn. One 'branch' of my family even uses them as ice-cream bowls.