Unlike some exchanges and brokers, we lock the crypto rate at the time of purchase. What this means is that you will receive exactly the amount of bitcoins you ordered - regardless of any fluctuations that happen between placing the order and receiving the coins.
In the past, we have tried locking the order to the market rate instead of the crypto rate - like Circle and CoinJar - but found that it created a lot of confusion and misunderstandings. The analogy people told us is that you're ordering a pair of purple shoes only to discover black ones arriving at your doorstep and a note saying "the trend has changed since you placed the order". If you like black then all is dandy, but you may not - and in our experience, many didn't.