If you have a new HP inkjet printer you should take a look the new service offered by HP called Instant Ink.

Once you sign up HP sends you a welcome kit with cartridges that once installed will link your printer to your Instant Ink account. When your printer starts getting low on ink your printer will order cartridges to be delivered to you before your printer runs out.

HP Instant Ink Registration

Using the link above will give you a free month of service.