Invoicing is essential for businesses of all sizes and it plays a vital role in getting you paid. It sounds you have QuickBooks Online Accountant, which gives you additional features for managing the subscriptions of your clients. I'm happy to inform you that it's just as easy to manage your one-time customers, and you have access to the same great features as a regular QuickBooks Online company. I can guide you through this.
You can set up a client whose subscription you aren't managing is through the Sales tab of Your Books, but the easiest way is directly on an invoice. It just takes a few easy steps to do this:
Click on the + New button and select Invoice.
Click the Customer dropdown then choose + Add new at the top of the list.
Enter as much detail as you'd like then click Save.
Create the rest of the invoice as you normally would with a Product/Service, an Amount, and any relevant Sales tax.