I wouldn't feel guilty at all. If they ask, tell them. I don't think you need to feel ashamed or hide that it's not from scratch. If your cakes look good & taste good, that's probably all they care about. When I've had to purchase cakes before, I've never even wondered nor cared what they use. I'm there for the convinience of not having to do it myself. Even though it may be a doctored mix, you've still provided a valuable service -be proud!