I will only buy online if it is a great deal and includes free timely shipping, but a rebate is an added bonus and definitely encourages me to shop online. I belong to both Ebates and Great Canadian Rebates and use them both regularly. Why not just maximize savings on one? Because they sometimes have better deals than the other one. For example, I received several promo gift cards from shop.ca last year. In my first purchase using them, I used Ebates as they offered an amazing 30% back on the purchase. Since the purchase was almost free using a free gift card, I made money on the purchase. Ebates later dropped the rebate to 20% then 10% so on my last purchase, I used Great Canadian Rebates as they were offering 15% at the time.
Ebates pays by cheque or paypal quarterly. Whatever you earn in a quarter is paid out on the 15th of the second month following the end of the quarter (so October-December will be paid on February 15th) but only if you earn over $5.01 before bonuses.
Great Canadian Rebates bases your cash back on your individual purchase dates, not per quarter. So anything you earn will be paid on the 15th of the month after it becomes 45 days old. So if you bought in October, you would get paid in December, much quicker than with Ebates. They offer Amazon, paypal (over $5) or cheque (over $20) for payments.
Both sites offer occasional coupons and with some of them, you earn rebates on the amount before coupons so you can further maximize your savings – be careful using coupons not shown on their sites as it may mean your rebate is refused (usually in the case of referral coupons).