When a customer purchases items on a regular basis, instead of creating a new sales invoice from scratch for every sale, it may be convenient to use the Clone or Copy to buttons to replicate a previous invoice:

Everything is carried forward onto the new invoice, including quantities, unit prices, discounts, and tax codes:

This may not be completely desirable if your item prices have been updated since you issued the prior invoice. But there is a quick way to remedy the problem. If you need to refresh the unit price of an inventory or non-inventory item, click the X inside the Item field to clear it. Then select the same item again:

This refreshes the Unit price field to the current sale price. Qty and Discount fields will not change. But Tax Amount and both Total fields will recalculate:

As always, any aspect of the new invoice can be edited before clicking Create. And the same procedure works when editing an invoice that has already been created.