There's probably a risk of making this issue more complicated than it needs to be, but here goes:
I'll cut a window in a customer's board, but the charge is size-dependent. I could make a long, drawn-out case for this, but that would be silly. Basically , I have to allow for the risk that my Mat Maestro will go berserk and I'll end up replacing the board. If I'm cutting a piece of glass for someone, and the glass cracks, I'm probably going to end up replacing the glass, so I charge for that risk too.
I'll sell anything in the shop, including full or partial sheets of matboard, that I don't need soon to complete an order. (If one of my teens is hanging around here not doing anything useful, they are at risk of being sold. Nobody's offered yet, though.) If it's a surplus piece or a board I don't normally want to stock, I'll sell it at close to my cost. Otherwise, I have a fixed mark-up from my cost.
There. That wasn't so bad, was it?
Oh wait! That reminds me of a story. Years ago, I bought a new VacuSeal press. My sister happened to be in town and needed something mounted. I had the press on and did it for her immediately as a family courtesy. Out of curiosity, she asked what the normal cost would be for that mounting. When I told her, she gasped. "But it only took you 5 minutes!" I reminded her that it took me 5 minutes 'cause I had a $4000 mounting press. She's not stupid, but she said, "Well you don't expect your customers to pay for it, do you?" (That one's for you, Bob Carter!) I hope to have my customers pay for my Mat Maestro before it completely craps out.
Ron