When an invoice is generated and paid via account credits, the tax is not added. Why?

In Exclusive Tax option, discounts and account credits are applied first on the price, after which the tax rate is applied. On the other hand, when Inclusive Tax option is chosen, the tax is already a part of the plan price. So the credits will be applied on the final price, which is inclusive of taxes.

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New Solution-

This happens when the taxes are inclusive in nature. In this case, the tax is already applied to the price of the plan, so only the account credits get used to pay the invoice. In exclusive taxes, the discounts or the credits get applied on the plan price and then the tax gets applied.

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