Just a quick help page to ensure that you can apply your coupon codes prior to checking out. As mentioned, I cannot apply codes retroactively.
Please note, some browsers may be different, I tested this on Microsoft Edge and an Android smartphone using Chrome. The general process of looking for the box to enter the code will be the same.
Ordering from a Desktop
After clicking checkout from the cart page, there will be a text box on the right hand side of your screen where you enter your coupon code. You may apply more than one, but they must be entered one at a time.
Ordering from a mobile device
When you are viewing your cart, you will see a bar at the top saying "Show Order Summary", click this drop down and you will see the box to enter your coupon code. You may apply more than one, but they must be entered one at a time.
I hope this helps!!