Two 33¢ stamps are NOT necessary. That's either an error on Kellogg's part or an oversimplification based on the assumption that most people only have that value at home.

However, I'd check the accuracy of your scale. Unless you have a weightless envelope and shaved more off the coupons than the dotted lines suggest, 5 of them (per the official scale at my local PO) take you over an ounce and into 55¢ territory. If you send in multiples, it's another 22¢ for each group of 5, at least through 20 coupons for $1.21.