Hi there, barrywbrewer.
There's a slight different on how to set up and use the percentage of subtotal in QuickBooks Online.
Here's how to set up an item:
Click on the Sales from the let navigation.
Select Products and Services.
Click on New.
Choose Service item.
Enter the item information.
In the amount field, enter the negative amount (if it's a discount) and enter the positive.
Click on Save and Close.
When creating a transaction, add this item. Here's a sample screenshot:
In case you need to view an article about applying discounts, here it is: Apply a discount on an invoice or sales receipt
I will make sure to connect and get back to you if you have other questions.
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