marlin,
first, are you currently building, or are you flying?
i called a tax guy that works for the IDOR, and he told me that i can pay the tax using form ST-44 (downloadable from the IDOR website) for each subkit that i order. it's not called sales tax unless you purchased a previously built RV in the state of IL. your parts are the subject of Illinois Use Tax, and you have until the end of the second month after purchase to pay it without penalties. i've sent in $448 for use tax on the tail and wing kits. keep the record of payment, and in the end when it gets registered, you've already paid your taxes.
if you've purchased a previously built airplane, just add up the cost of the kit, engine, and avionics, and that's what you pay on.
if you've built, but are just now paying, be careful of penalties...
