Hi, and welcome to Apple Discussions.
They probably would, especially if you buy it from them.
Why not choose your closest store from this list:
http://www.apple.com/retail/storelist/
and call them to see?
Apple usually charges a premium for RAM. You may find that you are quite capable of installing RAM yourself. Check out this Apple Knowledge Base article on installing RAM and see if you think you're up to it:
http://support.apple.com/kb/HT1651
Apple does consider it a customer-installable part.
You can probably find less expensive RAM for the MacBook at
ramseeker.com. It doesn't happen often, but once in awhile RAM fails, so be sure that you buy RAM that has a lifetime warranty.
Good luck!