Amazon offers the Gillette Fusion ProGlide Power Razor Cartridge 6-Pack for $19.12. Add it to your cart via Subscribe & Save to drop the price to $18.16. With free shipping, that's $3.03 per cartridge and around $10 less than what you'd expect to pay in your local store. They feature five blades and a precision trimmer on the back for edging.
Be sure to cancel Subscribe & Save once your order ships or you'll be billed for subsequent shipments.