Totally get this (it's such a weird spot to be in). Friends asking for "just materials" cost forget that our time and skill have value too, you know? Had a pal ask me to bake a huge fancy cake once for basically nothing, and it honestly felt like more work because you want it to be perfect for them. Ends up feeling like a whole lot of labor for a "thank you" and some leftover frosting. Kinda makes the whole thing feel icky, even when you want to be nice. Setting a small friends-and-family discount feels way more fair than working for free.
Seriously, just leftover frosting? @willowgarcia, that's hard to believe. It shows how some friends don't value your time and skill. A small discount is a fair way to handle it.