At CompAndSave, buy one refurbished HP 564XL-Compatible Inkjet Cartridge 4-Pack for $23.96 and get a second one for free via coupon code "DN4BOGO". (Add two 4-packs to your cart, then apply the coupon.) Shipping adds $3.99, or spend over $50 to qualify for free shipping. Each 4-pack features one each of black, cyan, magenta, and yellow ink cartridges. A 1-year CompAndSave warranty applies. Deal ends May 25.