I understand that you're angry ... but you just wasted a stamp ;(
By any chance, do you have a copy of the coupon? Or even if you didn't .. send a letter directly to the OxyClean company.. Tell them that you didn't see any requirement that you buy the 40oz size of detergent and that you think you're entited to the rebate. Be polite, but direct. No guarantee, but they could decide to send you the $$ anyway. (I did that with a well-known company when their toothpaste rebate didn't show up after 3 months. They sent a check and an apology saying they'd had problems with the rebate company)
Rebates can be profitable, but they also can be a royal pain ... I've read that many are set up to make you fail. A suggestion: Make a copy of everything you send (coupon, forms, barcodes, etc) and hang onto it till the rebate comes in. (I throw all my stuff into a folder and check it periodically.) If there's a problem, you can refer to the material and appeal .. or take it to the company. (Oh, yeah, I now staple the UPC to the form so it does't disappear ;)