I ordered 10 or 11 times between Thanksgiving and Christmas from Buy.com without a delivery issue at all with the free 7-9 day, super saver shipping . Some of my items were received within 3 days of ordering and these were the super cheap Kingston USB drives, SD cards, MicroSD cards, etc., that I am pretty sure were in high demand during the holiday season. The only one that took about 10 days to get to me was the second micro SD card and they said it would ship in 3-5 days and they did, (received it 10-12 days after ordering). I had plenty of time to send in all my rebates.
If the website says ships in 1 or 2 business days and you have 30 days to send in the rebate, I wouldn't worry at all. Now if the item states, ships in more than 2 weeks, I might reconsider. You can always cancel before the deadline.