$20 sounds awfully high to me. I give them away myself, but if I charged, I would ask no more than $5. That would not include postage or fabric of their choice, of course. There was a time when the suggested cost was to charge $1 for every 1000 stitches, but I’ve seen designs that have double digit thousands stitches, and then that formula becomes outrageous.
just my feeling on the matter.