I second what Sonoran said, DE is NOT a treatment for lice, mites, etc. If you have enough mites to NOTICE them, you need to treat them. Here are some options: pour on ivermectin, injectible ivermectin, permethrin sprays/dips, flea/tick shampoo, frontline spray, sevin dust, permethrin dust, etc.
It seems like the bigger the crest, the better the cushion, the easier it is for the mites to hide out and set up shop. Unless a bird had lice bad enough to have scabs I wouldn't ask for a refund. Mites/lice under control could have gained a foothold when the bird was stressed during shipping. Eggs could be stimulated to hatch by temperature changes in shipping. I would love to think I could be 100% mite free all of the time, and it's unrealistic. I haven't had lice yet, but with the wild birds, the fact I show, etc it might pop up some day, at which point I will treat. The main thing is to walk through and check your birds for them, if you find an infested bird in one pen you might want to treat all the birds in that pen.
I've had good luck keeping the scaly leg mites away, but the little buggers are a PITA.