Best Buy used to have something called MAF, Merchandise Authorization Form, where you could pay for something and then pick it up when it was received back in stock. This way you could use discount coupons or take advantage of special financing offers applicable on that day. They still might do this.
The rainchecks are for advertised merchandise whether it is actually on sale or not. Non-advertised sales, discount coupons, ect. don't get a raincheck if out of stock. The rainchecks are a way to prevent bait-n-switch tactics using ad circulars. Of course with all the main in rebates lately that require a purchase date between a certain time frame the only thing a raincheck guarantees is the before mail-in rebate advertised sale price if the item comes in after the rebate period, very likely.