Our apparel is printed to order, predominantly by local companies in the US. As a super-small business, this is the only way we're able to offer such a fantastic variety of designs on all kinds of rad things. (Considering we work out of our garage, even bulk buying one t-shirt design in quantities and number of sizes we would need to would fill up more than half our workspace.) One major issue we take with print-to-order apparel is that we are limited to the products these print companies decide to offer. Which includes sizing.
We regularly reach out to our print companies to apply pressure to diversify their sizing and "curvy" (read: plus size) options. Unfortunately, underwear is one category that has so few options available, that we hesitated to even add them to the shop at all. We are happy to offer panties and boxer briefs with our original designs, and made to match your gear, but we'd be even happier if we could offer up to a size 6X.
Our current best option outside of straight sizes are the boxer briefs, which go up to an industry standard mens 3X, or a 45" waist and 52" hips.