So we tracked it down and the person who shipped them got them weighed at USPS, confirmed postage, then loaded them all into the exact same envelope and shipped. The issue appears to be that the larger size t-shirts due to weighing a smidgen more offset the cost. It shouldn't be a $1 though, she really dug into it and the spread appears to be .38 .. Regardless it sucks, if you got dinged hit me up and I'll make it right.
If we do it again, lesson learned. No problem putting this together, be cool to do it again with another design, maybe a non-HBT design for brewing related.