When it comes to client-related finances, the discounts and surcharges can be applied in two ways:
To a task
To a receivable
While creating a client invoice, you need to take into account the place where the discount/surcharge is applied. Otherwise, it won’t appear on the invoice.
For discounts/surcharges applied to tasks, click the Add one item for all tasks or Add an item for each task button.
For discounts/surcharges applied to receivables, click the Add an item for each task receivable button.
As a result, the invoice will show the discount or surcharge applied.
The name of the discount/surcharge will appear on the invoice only when the Add an item for each task option is selected and the Client invoice receivable item name expression includes the receivable.amountModifiersDescription}tag. To check or change this expression, go to the 
Configuration menu > Templates > Expressions. To learn how to edit expressions, see the Templates - Expressions article.
If you use the Add an item for each task receivable option for a project where a discount or surcharge was applied to a task, the discount/surcharge won’t be taken into account in the invoice. However, if you use the Add one item for all tasks or Add an item for each task option for a project where the discount or surcharge was applied to a receivable, the discount/surcharge will be taken into account in the invoice despite its name not being displayed.