After much consideration, these are the 10 blocks I've come up with:
TNT
Bookshelf
Cake
Stone
Cobblestone
Cobbled Deepslate
Hay Bale
Oak Planks
Birch Planks
Cactus
The Stone, Oak Planks, and Birch Planks are there to be spacers for letter block signs from the other merchants, and so could be added to them and have three other blocks added instead:
Block of Diamond
Grass
Dirt
Each mini-block head should cost 5 cookies, as they are mainly for decoration and not construction, but can be used as detailing if people want.
Thinking is dangerous, but so is everything else when you put your mind to it.