Staples is $2 per cartridge up to 10 a month, at the end of the following month you get a email coupon for $20 ( if you returned 10)

I tried office max and they have a catch I dont know about office depot, but with max you have to also SPEND an amount equal to the amount of your returns in order to get the $2ea ( or maybe they are $3)

but if you want an even better deal than you found already... you can buy empty cartridges on ebay for around $200ish for 800 (prices vary a lot) - when turned in that becomes $1600 ( not that i ever tried that- or at least not that I would admit to it)