Once you make the purchase it is done. The vendor has 4 days to process your order and in that time he could cancel it. If he does you get your coin back. 99% of vendors will not cancel an order just because the btc price has fallen. Yes it happens, but again rarely. And for something al cheap as this (1 btc ) I do not think you have anything to worry about. But since your question was only if you make a purchase and it is on processing and the btc price go's down in value would your price be locked in...and the answer is yes....of course it would be.