The link you have provided clearly states that the free cartridge offer has to be added to the appropriate Compatible Twin Pack (black and colour cartridges) purchased for your printer in your shopping cart.
The offer doesn't apply to ..."Original or Remanufactured cartridges".
It's still a free cartridge, but you have to buy two compatible cartridges as well.
However, even these compatible cartridges on offer for my Canon printer cost £5.75 each or £17.95 for all four, which is three times what I pay at present for Jettec equivalents from CPC in Preston.
So even a "free" cartridge would make it a pointless exercise to make a purchase of either choice, despite free delivery.
Moreover, judging by the cartridge illustrations on the website you have provided, you can buy the same brand ones at click here for two-thirds of that price.
A pack containing the four cartridges needed for my printer is priced at £10.99 including VAT (£17.95 on your website).
I would suspect that it would cost much less than £1.95 to post an individual "free" inkjet cartridge as they are very light - in any case the postage is approximately the same price I pay for Jettec cartridges.