PC2 disk buying puzzle

[PoorOldSpike]

Hi, I bought PC2 by download last year and now want to buy the disk but I don't understand the checkout screen.
Look, the disk should only cost me £10, but when I click to buy it, it also adds the current £15.99 download cost to it and wants to charge me £25.99.
Like I said, I already bought the download last year and now only want the disk for £10.