This article covers 
The coupon is not applied to the tax amount. Why do I not see a tax line item when I apply coupons?

Understanding how coupons and taxes work on Chargebee’s checkout page.

Why is my coupon code not being applied at the checkout?

Error “Invalid coupon code”

How to set coupon code field on the checkout?

Summary

In Chargebee, discounts are applied before applying the taxes, if any, and taxes are applied based on the Billing address which will be collected on the following pages in the checkout. Hence, on the last page, the tax amount will be seen as a line item. 

Checkout page before Billing address information is added:



Checkout page before Billing address information is added:

Here's a screen recording for your reference

You can enable an option to allow customers to apply the coupon code in the checkout through Settings > Configure Chargebee > Checkout and Self-Serve Portal > Configuration > Enable 'Allow customers to add/remove coupons'.

Note: Make sure to Publish the change after enabling the setting.

When you apply a coupon to a checkout, you might come across an error “Invalid coupon code” as shown below:

This is because the coupon duration might be configured as "Forever" or “limited period” and hence can not be applied to One time checkouts. Only coupons with a duration of “one-time” can be applied to the One-time checkouts.

Related articles and Documentation

Why coupon with the full discount is asking for payment information?

Difference between coupons and manual discounts