Symbiostock and WooCommerce have a built in coupon system that permits you to create, edit and delete coupons, setting limits on them as per your requirements. Customers can enter these coupons during checkout to receive discounts on their orders.
Refunds will depend on your chosen payment method. For PayPal payments, you can enable the API credentials within your WooCommerce settings so that WooCommerce can initiate refunds on your behalf. If you don’t use the API credentials (which may require a PayPal business account), you will have to manually log in and refund any cancelled orders via PayPal. Enabling the API credentials is very easy, and can be done right in your PayPal account under Selling Tools > API Access > API Credentials/Signature.