This seems really odd. I've sold and mailed gift cards to plastic jungle (which suggests on their website that you insure them) have always insured them. I've insured many packages/letters I've mailed and I've never once been asked what was in the package or been given restrictions. I would call a different post office (so you don't get the same clerk) and ask if there is a list of mailed items that cannot be insured. It almost seems that the person who might have stolen your gift cards was the clerk you handed them to at the post office.
Edit: Nevermind it looks like everything worked out, still though I always insure valuble things in the mail.