Docs
I use metered billing in the current term and I cannot add coupons to the Pending Invoice. Why?
Summary
Coupons should be added at the Subscription level for them to be included in the Invoice. When you regenerate an invoice it will no longer be in the Pending state, so ensure that you add all necessary charges before regeneration. This way, the invoice will directly move to Payment Due or Paid status.
If the Coupons were added after the Pending invoice was created for the subscription then,
Delete the Pending invoice (after backing up if necessary)
Make changes to the subscription (add the Coupon)
Verify and Regenerate Invoice for the current term
You can only apply multiple coupons of the same type from Chargebee's' UI.
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