The bit of profit you are screwing them out of generally winds up being very expensive if a warranty is needed. You want me to install Moog ball joints that you are supplying? No problem, when you come back in 5000 miles because you have DW, bring another set with you and you're paying me to do it all over again.
All the gear shops in this part of the world are the same, supply gears and locker, no problem. Have a problem, you get to pay them to remove them, then you get to pick up the parts, go get them warrantied, and then pay to have them put back in. Don't dilly dally or storage fees can start to apply.
OTOH, they supply the parts, drop it off and pick it up all fixed for no charge.