Extra Postage Required

I recently bought two birthday cards to be mailed. As if it isn't hard enough to find that card that is just right, envelopes come in all different shapes and sizes. Some of these are marked "extra postage required". SO what is that? How MUCH extra postage and better yet WHY? As if it really costs USPS more to mail a square card?! And I am forced to put two 44 centers on because a) I barely keep stamps on hand, let alone various amounts and b) the last thing I want is an already late b-day card coming back to me 5 days later marked "insufficient postage". I'm so sure it doesn't cost the USPS anything to send it back to me either.