To redeem credits, copy the code given in the "Referral Credits" section of your dashboard and paste it into the "Promo Code" section of the checkout page. Only one can be used per order.
Only one credit can be used per order. However, there is no limit to the amount of credits you can collect and how many you can spend in a given time period.
No, credits are non-refundable.
After the initial purchase with the referral code, it can take up to a day for it to process and show in your dashboard.