I wouldnt suggest sending them from an exchange as there is only a 1 hour window to complete the purchase. You only need 1 confirmation which is the good part and Bittrex is pretty good about releasing funds quickly but just a heads up. Sometimes exchanges batch withdraw requests to reduce the load on the server so if you end up getting stuck in a batch that doesnt run for more than 30 minutes and then it takes another 30+ to confirm your send then you will miss your window... then need to contact support to get your order processed. In that time you are waiting, if the round changes then you will only get the new round bonus rather than the one you actually bought in. I sent my BTC purchases from Coinbase and things were confirmed within 20 minutes. Also make sure that the amount that you are sending has the tx fee added on top of the amount needed for the purchase and not deducted from the amount you are sending.

I like coinbase because it adds the fee on top of the amount you are sending and it will not cause any problems.