A bookplate is a fancy label or card that you affix to the inside cover of a book. Traditionally, this was used to identify the owner of the book ("From the library of Andy Smith," for example). It is also offered as a way for an author to provide a signature for a book, without being present to sign the book itself.
I will never actually be in the same room as the books being shipping internationally. They will go straight from the manufacturer to a shipping warehouse. And all of the book sets will be individually shrinkwrapped to protect them from moisture during shipping, so then signing the books would require opening each set.
A signed bookplate was the best approach for this set.