As I understand it's the last chance to get it for the pre-order discount price, as of tomorrow it'll go up to full price (until Steam discount it!).
Edit : I keep meaning to do this.. Paypal wanted £7.81 with their exchange rate.. so I'm making a note of that to find out if it's cheaper to pay with my credit card in dollars, or take Paypal's rate. I suspect it's a matter of pennies.
Edit : It's already showing as an authorisation for £7.57. Not a big deal, but it's the principle that counts and yet another Paypal sharp practice.