In the case of an underpayment, the system will detect this and allow you to send a second payment for the difference and complete your order. In the case of an overpayment, we will issue you a refund for the overpayment amount, less any conversion and transactions fees. (This works out to be around 5% on average.) You can visit our full Policy for Refunds on Crypto Overpayments page for a full breakdown on how we handle this process.