I think that detail about the billing USSR for the return shipping of the plane is propaganda meant to humanize the US/Japanese side and dehumanize the USSR sid

I think that detail about the billing USSR for the return shipping of the plane is propaganda meant to humanize the US/Japanese side and dehumanize the USSR side by showing that we have a sense of humor and they don't. USSR could have replied by sending an invoice for the rental of the plane, reassembly of the components, and the intellectual property licensing for all of the technical data obtained. All of that would no doubt ought weigh the cost of crating and shipping the plane by orders of magnitude. Not that either side would ever have considered paying, of course. In reality, some entirely secret concession could have been made that we might never find out about unless it was documented and is some day declassified.