I've just rung my local (Chester) Staples store to check stock of the HP 364XL ink cartridge.
The cheapest that I can find on the web is £13.94 inc VAT.
The deal that Staples are offering allows you to take (up to) 5 empty / used cartridges and receive (up to) £10 discount for each new 364XL bought.
This brings a £15.99 cartridge down to an impressive £5.99!
The very informative man that I spoke to told me that they can do (up to) 5 individual deals, each saving £10 for each batch of 5 empty cartridges received.
He was quite specific in saying that the empty cartridges MUST BE GENUINE HP CARTRIDGES and that REMANUFACTURED CARTRIDGES WILL NOT BE ACCEPTED.
The deal is on ALL HP cartridges (not just the 364 or 364XL).
I have noticed that the store prices seem to be higher than their online prices so I suggest that you contact your local store to check stock and prices before you set off to avoid any wasted journeys.
I'm not sure how long that this offer is on for but I'll find out and post an update.
If anybody intends to go to the Chester store, please wait a short while until I've had a chance to get there!!