don't bother ever buying anything, just download or get a copy of whatever linux software you want, and if you like it, make a donation to the company or charoty that is responsible for that distribution. You don't say where you are in the world, so recommanding mailorder is difficult, i heard of a good US based one, but cannot remember what it is, here is a similar service in England:
www.yourlinux.co.uk these guys download full linux distributions and i think some other systems and software using their broadband connection, then sell the CDs at cost price, a few quid per order and so on.
If you have broadband yourself, just download a full distro yourself, try going to
www.linuxiso.com for a few ideas on which one to get, and also for some installation documents.
The good thing about getting and installing linux is that nobody will charge you for it, you just use it, then you make any contributions, money or skills, that you want to. Good luck, and i hope you enjoy using linux!