@2muchsoysauce4allthisrice,
I only want to stress that
@SfGiants_fan1 should contact his preferred CA FFL about plausibly accepting a transfer on a lower receiver assembly. You definitely make valid points and I only wish that CA FFLs would follow them universally. (In my experience, they often don't.)
Now, the transfer is either (1) originating from a private party (an individual) via an out-of-state FFL to a CA FFL on behalf of
@SfGiants_fan1, or (2) originating from the inventory of an out-of-state FFL to a CA FFL on behalf of
@SfGiants_fan1. In this case of the former, no invoice is required. Rather, the private individual who currently owns the lower receiver assembly merely needs to include a copy of his state's identification and a hand written note stipulating that the he's selling the lower receiver assembly to
@SfGiants_fan1. In the case of the latter, the considerations are different. Since such a transfer is a species of retail sales, wherein a serialized item is being disposed, an invoice for the sale is required. If
@SfGiants_fan1 offered a trade with the hope of off-setting the retail cost of the lower receiver assembly, the out-of-state FFL should reflect the trade value in the form of a deduction or subtraction to the retail price of the lower receiver assembly. Ultimately, only
@SfGiants_fan1 and the out-of-state FFL know what the true value of the trade was.