If some customers are invoiced but others are not, how do I process the payment for those who don't have an invoice? Do I just code it to "Sales of Product Income"?

There are two-ways on how we can record those customer's payments, phoenixpetsuppli.

We can create either an invoice or we can directly issue a sales receipt.

Here's how you can do it:

1. Click the + New button.
2. Under CUSTOMERS select either Invoice or Sales receipt.
3. Select a Customer.
4. Enter the PRODUCT/SERVICE, QTY, and AMOUNT.
5. Select either Save and close, Save and send or Save and new.

In case you're ready to reconcile the transactions, you can check this article: Reconcile An Account in QuickBooks Online.

Don't hesitate to get back to me if there's anything that I can help you with.Stay safe!