I don't think it voids the warranty. If you're unsure, best to call the company and ask to verify. Whenever I've done rebates like that since they typically ask for a copy of the packing slip or invoice I make a copy of my original and place the cut out upc label in a blank area of the invoice. That way I still have a copy of the upc for my records.
It's been awhile since I've done a rebate on a gpu, but I think they're similar to other components. If you look at the side of your mobo box you'll see a white square (usually a sticker) with all the upc info - the barcode, numbers, make, model and serial number. Cut out that entire label. There may even be a dotted line around it showing you where to cut. That's what I did for 2 gigabyte mobo rebates around xmas 2014 (black fri sales) and both went through just fine. Got both rebates. They asked for the upc label off the box, copy of the packing slip and a printed off rebate form. The rebate form has a mailing label on it, cut that out of the rebate form page and tape it to the front of an envelope when you mail it in. That way you're sure it goes to the correct address/dept.
I've never done mir's for sapphire or evga, just gigabyte and xfx. Just make sure to keep on top of the rebate though, they usually use a 3rd party rebate processing center. I didn't get my rebate when I should have for my xfx gpu and contacted the company with all my info. Got the rebate within about a week. Sometimes things get lost in their accounting system. So long as you follow the directions literally (treat it like you're 5yrs old, step by step) there shouldn't be any hassle. Not saying it's so simple a child could do it, but I think a lot of issues getting them processed happen when people get creative and think 'oh well, they'll figure out what I meant or what this is'. Best of luck.
Facebook
Twitter
Instagram