Add twice a credit of 500 prints, which cost £25 each. However when going to checkout you can enter promo code XXSAVE20, and you get £20 off, so £30 for 1000 prints (3p per print).
If you want just 500 prints, you can enter code XXSAVE5 and get £5 off 25, which will cost you £20 for 500 prints.
You don't have to use all prints at once, you can keep using it in multiple orders. Expiry date is just for the codes.