Is there a way to allocate a specific product/service to a customer so that each time that customer is added to a sales receipt it remembers the product they bought?

Hello, melindai.

It's definitely time consuming to manually enter the item they bought every time you create a sales receipt. Using the Recurring template will help you save time. Recurring template is duplicate of existing items that are automatically generated by the system. I'll show how to use it.  

1. Go to Settings.
2. Under Lists, choose Recurring Transactions. 
3. Click New. 
4. Select Sales receipt. 
5. Enter the information needed. 
6. Choose a Type: Scheduled, Unscheduled, or Reminder.

If you're looking to learn more about the feature, this article has all the details you need: Create recurring transactions in QuickBooks Online.

Our sales and customers topic can also help you create and customize sales forms, manage your clients' profiles, and more. Browse the helpful articles. 

Let me know if there's anything else I can do for you. I'll be happy to lend a hand. Have a great day!