I got bitten by the weird sizes one when returning a manufacturer's product warranty registration card. Because the postcard was too close to being square (rather than standard postcard shape) it cost more to mail than if I had enclosed it in a standard-size letter envelope and just mailed that. The annoying thing is that I'd gone to the official usps web site to look up the correct postage and I'd followed the instructions to the letter. You couldn't get to the page explaining about odd shapes unless you already knew that you needed to go there.