Programming a replacement remote for a 2003 Mazda 6

I am about to purchase a replacement (ebay) for my second keyless entry remote. The specs match up completely and I'm not even certain if it's a MAZDA part or an aftermarket one. Anyway, how easy will it be to program? When I enquired about replacement about a month ago, the dealer quoted a replacement part cost of about $50 and I think a programming charge of an additional $20 (though that might have only been a charge/quote only if I brought my part own to them). Obviously, I'd like to avoid the extra charge and the replacements on ebay look good...and they're cheap. Any ideas re doing this programming myself? Some of the ebay sellers offer "instructions upon request", but I thought I might get some info re this in advance (and a sense if their offers are likely to be legit). Thanks a lot.