Here's how to apply a code to your upgrade:
Head to Settings > Plans & Billing > Choose the plan that works best for you > Upgrade
Click "enter promo code"
Apply your coupon code in the "Promo" submit box.
Enter your payment details
Select the "Pay Now" button.
Choose the plan that works best for you and select "Upgrade".
A payment page will pop up.
Make sure to click the "enter promo code" and input your coupon code.
Contact Polymail and we would be happy to help you out!