Unless you are buying a new Apple Computer and value your time and effort over money which is the only reason I can see on why you would want to add memory from the Apple Store, I would buy memory from a third party and install the Apple memory yourself.
Why? Buying memory from pretty much any memory store will be cheaper than if you bought RAM from Apple directly. To their credit, Apple does offer you the ability to buy the computer and then fill Mac with RAM at time of purchase online. This means you don’t have to replace RAM, as you would have to do if you were to buy your Apple Computer separately and then upgrade, as most of us do.
The downside of buying your Apple computer, and then buying RAM from a much more affordable source is that you are left with the original RAM to deal with. You can sell your memory which will offset the cost of your RAM purchase, but there’s more effort and steps involved if you decide not to buy RAM from the Apple Store.
If you’re buying a new Mac from Apple, and the extra cost is no big deal to you, than just order your new Mac with the RAM you want and be done and happy with your new Apple Computer.
If you are already in possession of your Apple Computer and want to upgrade the RAM, and want to save money on RAM then the smart thing to do is upgrade the memory yourself and deal with the factory memory that you pull from your mac. You can sell the ram on eBay, make a keychain, give it away – the choice is yours, but if you’ve already got a Mac to upgrade, then no, do not buy RAM from the Apple Store.