Using ShopDiscover online
Other rebate sites may also say that a code that is not provided by them will void the rebate from their site. That may or may not be the case. Many of us have successfully used a code not provided by the rebate site (e.g. urlhasbeenblocked) and received both discounts. But this is hit or miss. They can always decide not to give you their rebate if they chose as you have been warned not to use an outside code. When making a large purchase, keep these things in mind as it may change how you wish to proceed with the sale.
Also, remember that Discover may say it gives 1% cashback on all purchases, but that is not true. Stores such as Walmart and buying clubs (e.g. BJ's) pay roughly .25%. AND, in order to receive the the full 1% certain tiers of spending must first be made.
Personally, I like Discover. I like their virtual numbers for online purchases. But it is necessary to understand the pitfalls so we can save the most on our purchases. I hope this helps someone who might have lost out on online discounts.