They have to be under 1/4" in thickness, which is the catch, but it can be done.

Your butterfly and a stamp is correct. Sometimes, because I weigh them, I need a Butterfly and two stamps. But because I have stacks of old USPS stamps, still good, I use a butterfly and slap on enough 10¢ to cover what it needs. I'm using up old stamps that I bought at below face, and I have some sheets of forever Butterfly stamps.

Pirate SHip sounds like a good idea, I'm going to look into that. My Brother uses Shippo.

ps Don't forget the new feature that USPS has introduced to screw up things even more than before. Your mailings, will not be postmarked where you drop them anymore. They will be postmarked when the are processed at the regional center. That means, I could mail a letter today, it could sit until Friday, and only then, get postmarked. Most of the time, who cares? But if you mail a bill, or tax document that needs to be postmarked by... it will be late. And we could be fined for that. Thank You USPS