What they charged was 286 x .0016 plus the 6171 satoshi that they add to all orders. So, 0.45766171.
Actually, I think the final 4 digits of your spend = your customer number. (Is 6171 your customer number) So, if you don't send them the right amount, they don't know what account to credit.
Whatever. I always make 100% sure that they get the exact requested amount and never had the slightest problem with that.