If coupons are a part of your business model, and easy way to enter them in PetExec is to create a product for the coupon, and add it to the customer's cart at checkout. Begin in the PetExec menu under Products & Services-->Add Product/Service.
Once you have added the product/service, you can simply add it to the customers cart at checkout.
From the Order Confirmation screen click the "Add Product Button"
From the Add Product/Service Screen
From the Order Confirmation screen you can now see the Coupon product, the amount of the coupon applied to the order, and the revised subtotal. You can then complete the checkout process. The customer will see the coupon on the receipt as well.