Can someone assist me with calculating interest on overdue invoices.

Thank you for contacting us for assistance with your overdue invoices, Raes. I can help with the interest added to your invoices.

In QuickBooks Self-Employed (QBSE), the feature to automatically add or calculate invoice interest charges is unavailable. However, we can add interest to your invoices effectively by manually calculating and adding the charges.

We can add a separate service item to overdue invoices and resend them to your customers with a note stating the additional interest charges for exceeding the net 30 payment term.
 

Here's how:

1. Go to the Invoices menu.
2. Choose the overdue invoice and select Edit.
3. Tap the second line in the Product or Service column to add more items.
4. Click +Add new.
5. Enter the name of the item, then add a description and amount.
6. Select an Amount type.
7. Hit Save.
8. Tap to Email to resend the invoice.
9. Press Send.
    

Additionally, check this link to add sales tax to your invoices in the future: Manually track sales tax in QuickBooks Self-Employed.
 

We are here to assist and ensure customers know the additional charges on the invoice. Please let us know if you have further questions regarding interest charges on your invoices. Our goal is to make sure everything is taken care of for you.