The tail end of a recent email from Choice Stationary this explians how it works.
"Don't bin that empty cartridge! When we dispatch your order and it is for a cartridge that can be recycled we will include a freepost envelope for you to do so. Plus if you order another cartridge at the same time we will give you £1 off. For more details please click here"
Thank you for choosing Choice Stationery Supplies and thank you for your valued order.