If you have a promo code for a specific product you can only apply it while ordering the product.
On the payment page, you will find the field to put in the code at the top of the summary.
- Click on the box
- “Do you have a promo code?”
- Enter the code and click on "Apply"
A seller can offer different payment plans, such as one-time payment, subscription, installment. It’s possible that the code is valid only for a specific payment plan.
If the discount applies, e.g. only for the one-time payment, it is automatically selected after clicking on "Apply".