Can I add 3% interest when the customer chooses to pay by credit? I want it to be done automatically every time the customers choose the credit option, is it possible?

Thanks for reaching out to us, @zvi-t-thesquadro.

Currently, adding interest automatically when your customers pay using a credit card is unavailable. You'll need to manually add a line item as the interest in your customer's invoice.

Here's how:

1. Go to the Gear icon, then select Products and services.
2.  Click New and choose Service.
3. Enter a name for your service item, such as Interest.
4. Under the Category, select Service.
5. Leave the Sales price/rate blank, then choose your Income account.
6. Once done, click Save and close.

You can finally add this as a line item for your interest if your customer pays using a credit card.

Just in case you want to run a financial report, you can check out this reference for your guide: Run reports in QuickBooks Online.

Please feel free to visit the Community again if you have other concerns about your invoice preference in QuickBooks Online. I'll be here to provide a helping hand.