So please censor this if it gets into the "not allowed here" / "illegal" territory and I'll never mention it again.... but...
I've always wondered if you could buy a magnetic stripe reader/writer and then use either an old bank card (and hope they don't read the valid date to close, though seems risky) or use an actual bank card that is still valid but you closed the account on or something.
Then just clone the gift card info into your bank card before your trip to the PO.
Actually... even better. There are cards like "Coin" that allow you to store up to 7 or 8 card on it and then still use a traditional magnetic stripe to read it. I believe those you can even load new cards on via a smart phone app or something. Now of course those are a bit pricey and eat into profit ($100 or so) but I wonder if that would be valid enough for a post office clerk... as long as the point of sale doesn't say "gift card" during the swipe or something...