Kevin:

Why not offer the plane as both short kits and full kits, with all wood needed? If the wood costs $15, you should be able to buy it for less in volume, say $12. The person buying it for $15 would have to pay shipping of say $6. Plus the convenience of having it supplied would be worth a few dollars. You should be able to add it to a short kit and charge an extra $25, netting you an additional $14 per kit. You may even sell more kits, because a lot of guys don't want to have to mess around buying both a short kit and then ordering balsa from a second company. So they buy a kit from someone else.