As I understand CA rules, when you first register the plane in CA, you have to pay the difference between the use tax rate and what you can prove you've already paid in sales tax. So if CA says you owe 8.25% and your plane is valued at 100k, you would owe $8,250. If you saved the receipts from Van's that shows you paid $2k in tax on the kits, and another $2k in tax on the motor, then you only owe CA $4,250. CA use tax is paid in arrears as well, so you don't technically owe anything until the year after you register the airplane. Register it Jan 1 2021, don't pay til 2022. Register it December 31, 2021 you still pay in 2022.