What is the best logic bypass or loophole you've exploited?

(This is not my story)

A few years back, an online store had this promotion where whoever spent the most money over a month would get free round trip airplane tickets to anywhere in the world. My friend (who's a fucking genius) found that one thing you could buy on the site was a gift certificate. So he bought a $25 gift certificate and kept spending it on another $25 gift certificate. So he ended up spending $25 on round trip tickets to Australia.