Good day, @danab88.

It's a pleasure to see here in the Community. I can share some information about allowing customers to add gratuity or tip in QuickBooks Online (QBO).

Currently, QBO doesn't have the option to let a customer add a gratuity/tip when paying an invoice. However, you can use the Tip tracking feature that is available in sales receipts or add a separate service item for tips on the invoice as a workaround.

To turn on the feature, here's how:

1. Go to the Gear icon at the top and pick Account and Settings under Your Company column.
2. Select the Sales tab at the left, then click the Pencil (edit) icon on the Sales From Content section.

3. Check the Tips (Gratuity) box, then choose My Team if you use tips to pay your employees or others. This will create a liabilities account for tips.
4. Confirm the template switch, then hit Okay and Save.

Once done, you can add a tip on the button of the sales receipt transaction. Check out this article for further guidance: Add tips to sales receipts.

On the other hand, here's how to add a separate service item for tips on the invoice:

1. Go to the Plus icon at the top, then pick Invoice under Customers column.
2. Select the customer and enter the necessary information.
3. Under Product/Service, click +Add new, then choose Service.

4. Name the service as Tip, the select Undistributed Tips under the Income account, then Save and close.

5. Once done, select Tip and enter the Amount.
6. Hit Save and send.

That should get you pointed in the right direction. Please let me know if I can be of additional assistance. I'll be happy to help. Have a great rest of the day.