You don't usually need to keep the original box or receipt to RMA hardware to a manufacturer. It's not like you're trying to return it to the store. They know they made it because nobody else does. You don't need to prove that it's actually a mouse of theirs.
A lot of companies are really awesome when it comes to replacements. EVGA has replaced a few graphics cards for me that have gone out way after their warranty ended, as a customary replacement because I'd never had to RMA it. I've never used Razer, because I don't really like their mice, but you ought to wait and see what they say.
Also, in the future, when possible, always keep your old mouse around (if it works). You never know when you might have to swap back for a while.
Using a regular mouse will kill actually. In pvp especially. Might have to go back to pve. Since it requires no skill and all.
http://www.logitech.com/en-us/mice-pointers/mice/devices/5750 is the mouse I use. Currently Logitech has a promotion going on for the next month that will give you half off any purchase from their site, so if you can find the promo code online, you could get that one for $35. They have some cheaper mice, but they're not as gaming oriented, and have far fewer buttons.