Ebates is an awesome site that too often I forget to remember when I'm purchasing online. When you hit up Ebates before you shop online (and use their links to get to the store you want to shop at) they will give you cash back on your purchase.
I realized this morning as I was about to make a purchase on CostCo's website that I had gotten an email from MyPoints offering me a points bonus if I bought a CostCo membership through them, which reminded me that I needed to check and see if Ebates offered cash back for CostCo online purchases. Unfortunately, Ebates doesn't have CostCo listed :( But, when I got there I saw that I had a big Cash Back amount waiting for me. I didn't remember hitting them up recently so I checked to see what it was and saw that (oh yeah!) I'd used them before I headed over to Dell to buy my new laptop back in December!
The typical % is between 2% and 7% and varies from store to store. They also do special promos at different times for holidays or events that the stores might be having.
I mentioned MyPoints, earlier, so I'll mention them again. They are a site similar to Swagbucks, in that they give you points for different actions (searching using their engine, clicking on links in emails, shopping through them). To be honest, I prefer Swagbucks as it takes fewer points to earn rewards. I earn a ton of points through Swagbucks by using their search engine, answering their polls, printing coupons through their site and doing other things that I'd be doing anyway. They also have some fun time wasting ways to earn points like watching SBTV (youtube) and playing games. Mypoints is a little slower to build, but I find it worthwhile to do both, and I redeem those points for giftcards to Amazon or Starbucks (or a number of other places).
For what it's worth I earned over $150 in Amazon gift cards last year from Swagbucks, so they by far win the race on earning free stuff. For that reason alone, I usually check them before I use any of the other sites.
Just for the record, outside of purchasing the membership certificate through MyPoints, it doesn't appear that any of them offer points or cashback for CostCo purchases. Oh well. I guess I'll just have to live with the fact that I'm already getting the item for about 1/3 of the regular price!