Apple will install RAM at any time - for a major fee (actually, I think they install it free, but they charge way too much for the RAM). Hard drives, Bluetooth, AirPort, etc. - these are all installable after the purchase.
However, given installation costs, it's often best to wait until you can afford what you want, and have it built to order.
If you know anyone even remotely computer-savvy, you'll save a ton of money ordering RAM from a reputable third party and installing it yourself (or, rather, via the acquaintance).
The Mac Mini (like all other Macs) can have more RAM added at any point after you've bought it. There's no need to send it back though it's very simple to do yourself and won't void your warranty in any way.
I think I remember reading that the hard drive in the Mini is also rated as a user serviceable part.
I think that the general concensus is that, on the mini, the RAM is not considered by Apple to be a user-serviceable part, and replacing it yourself will void the warranty.
I don't think that would bother me personally - just keep the old ram and swop it back if you need to send the machine back for a warranty repair.
If in doubt try ringing Apple to ask them.
Have a look at http://forums.macrumors.com/showthread.php?t=131633 for a discussion on the subject (and there's a more childish argumentative thread somewhere but its not worth bothering to link to it...)