You can still pull the thirds your self and have a shop set them up, in fact the shop that I use prefers that as you car is not taking up space in his shop.
A good busy shop will have and sale you the gears that you need saving you money as they make the mark up on the parts. most shops don't want you to bring in your own parts. You will save money on shipping as well.
Last year when I broke my ring and pinyon, I pulled the third and had the R&P replaced for $650 parts and labor I did not need a master rebuild kit because I did not have very many miles since the re-gear and it let go in my drive way.
I think you would be wise to find a reputable shop near you and just bring them your thirds
The only reason you would buy the R&P your self is if you are going to install them your self. Remember a shop is buying in bulk and getting a discount that you can't get.
It's also eraser for a shop to warranty parts that He sales you.
One last thing if there is a issue you don't want to have to ship them back.