I just found the fine print on their website, which states "We guarantee Welp-Line pullets to be 90% accurate to sex. If more than 10% cockerels, we will refund the difference between pullet and cockerel price. Claims must be made within 8 weeks of hatch."
I didn't know what the genders were going to be within 8 weeks of hatch, with some of the birds it wasn't obvious until they were quite a bit older. But I already got an email back from Welp, and they are going to honor the 90% (4 or more of the original 35). I lost around 1/3-1/2 of the original order to predators, but just in the birds I have left I still have 5-10 cockerels, I have to go get an accurate head count and make the final determination on a few of them.