2. I was required to pay a bond to US Customs to get clearance into the
US (even though, under the Customs regulations, there is no duty on
importing civil aircraft). That was an unexpected and onerous expense.

There are forms and using the wrong form will cause issues. I ran into something similar bringing an L-13 in from Canada a few years ago. At the first customs crossing I handed customs the wrong form (I had printed them all out just in case). This caused a requirement for a broker and possibly a bond, and being Sunday, the prospect of an overnight stay. We drove an hour west to the next border crossing and I handed customs the simpler form. He walked about, stamped the paper and asked me what I was waiting for. Didn't even ask about the trailer.

Not Mexico I agree, but I believe a bond only relates to a commercial sale or item for re-sale.

Frank Whiteley