Thanks for reaching out to us, @destination-marathons.
When receiving payments, you can only select the customer associated with the specific project. However, you can assign payments from different customers to the same project by creating a new one. Allow me to walk you through the process.
Follow these steps on how to assign payments to a project:
1. Go to the Sales menu, then select Customers.
2. Choose a specific customer then click on New transaction.
3. Choose Payment.
4. In Outstanding Transactions, select the invoice checkbox that the payment applies to.
5. Select Save and Close.
Follow the steps on how to create a project:
1. Go to Projects and select New project.
2. Fill out the Project name, Customer, Start date, and End date fields.
3. Click Save.
You may also read this article if you want to add an existing invoice: Add existing invoices to QuickBooks Online Projects.
Please don't hesitate to contact us again or simply reply to this thread if you have further questions about projects. I'd be happy to help.