We use Coinbase and BitPay as our payment gateway.
When you confirm your purchase by using Coinbase, Coinbase will use its current exchange rate to convert the amount of your order from USD to BTC.
OR
When you confirm your purchase by using BitPay, BitPay will generate an invoice by converting the amount of your order from USD to BTC according to their Bitcoin Best Bid (BBB) exchange rate (update every 1 minute).
BitPay consolidates market depth from multiple exchanges to provide buyers with a Bitcoin Best Bid (BBB) exchange rate. For details, please visit https://bitpay.com/bitcoin-exchange-rates.