Late fees are setup in QuickBooks Online directly, not Prima. Please note the below article is taken from QuickBooks Online help on 1/19/2021 and may have been updated since:

How do I set up late fees in QuickBooks Online?

To set up your default automatic late fees in QuickBooks Online you must be logged in as the Master Administrator or Company Administrator.  Go to:

  1. Select Settings ⚙️, then Account and settings.
  2. From the Menu ☰, select Sales.
  3. Choose Late fees. Check the box to turn on late fees.

You can specify the fee type, amount, grace period, and how it will show up on the invoice.

There are three different fee types to choose from:

Fee Type*DescriptionExample
PercentA percentage of the total invoice amount10%
Flat FeeA specified amount not dependent on the invoice total$20
Flat fee and percentA one time specified amount applied when the invoice becomes overdue. Plus a percentage of the total invoice amount applied after that if it remains overdue.$20 + 10%

$20 is applied when the invoice become overdue. The 10% fee is applied if the invoice remains overdue.

*All fee types are applied dependent upon the chosen frequency