Have had HP Instant Ink for a year now, it's fab and much better value than buying cartridges - you have a free trial, then sign up from £1.99 a month, allowances roll over and can be changed at any time. I'm still using my free months! They give a code to share - you and a friend both get an extra month's ink free if you use this code -
ry.hpinstantink.com/hvztZ