Some vendors do not want to display the coupon field in their cart if they are not offering coupons because they feel it encourages buyers to leave the checkout and go coupon hunting.
To address this, there is a theme option to show or hide the coupon field.
To toggle the coupon field:
1. Log in to DPD
2. For the store you want to toggle the coupon field display on, click Theme in the left menu
3. Edit your current theme
4. Find the Cart page options and open them by clicking on the title
5. Uncheck or check the box to toggle the coupon field displaying and save.
The change is immediate on your cart.