Prices change daily.
As far as I'm aware, just because a price for an item drops before you get your PC doesn't mean you get the difference back, in the same way that if the prices went up after you'd ordered you wouldn't be charged for it (this was most obvious when HDD prices were increasing dramatically each day) Note: I could be wrong.
Considering what Meds just said about free gifts/games in another thread, I think PCS wil just honour what ever price you ordered at. As Rakk said, the prices change often, and PCS probably wouldn't be able to operate efficiently if they were forever having to refund anything just because prices change.
I'd also add that PCS refunded the 2% to me without hesitation as I actually paid by a debit card.
If I could make a suggestion; I opted to pay by credit card but when I saw the message about the 2% fee I used my debit card details instead. I didn't think that I needed to change the payment type drop-down from 'credit card' to 'debit card', and it seems the PCS systems aren't intelligent enough to determine from the card number which type was used. I'm sure I can't be the first idiot who's made the mistake - perhaps there's some cleaning up on the process or page text which could stop anyone else making the same error.