Seems like a great price for a 30x20cm Canvas. Better yet, when you buy it with a 40x30cm canvas you save £5.00 making the total £14.99 delivered for the two! No code needed, discount applied at checkout.
Seems to be a discount for other sizes when going through to checkout, so have included those in the image below. :D
Just make sure you line up the image as you want it as that's how it will be printed. It's pretty straight forward and once your image is uploaded you will be able to adjust it on the canvas to your wants/needs.