When a customer uses a payment code at checkout with your webiste, any coin rewards earned during this transaction are only issued against the final sale price after discount. Furthermore, the customer will not earn coins for shipping costs and taxes. This logic ensures that customers will not earn "double rewards" on transactions where their coins are redeemed.
For example: A customer creates a payment code for $10 in coins. On the participating business' online store, the customer adds an item to their cart that costs $30. At checkout, when they apply their payment code, the final price is adjusted to only $20 + shipping cost and taxes. When the customer completes their order, they will only receive coins based on the $20 they spent.