Let me help keep the process of finding a customer transaction by its invoice number easy and simple, Lucy.
QuickBooks Online has the flexibility to search a certain transaction in the system. You can utilize the built-in Search feature in the program to find the entry using the invoice number.
Just click the Search icon and enter the specific invoice number. Here's a screenshot below for reference.

You can also find the said invoice from Sales Transactions page by pressing CTRL + letter F key on our keyboard altogether. To do this:
- Navigate to My Apps in the left menu panel and choose Sales Transactions under the Sales & Get Paid dropdown..
- From there, press CTRL + F key on your keyboard.
- Enter the specific invoice number on the Finder box.
- This will highlight the transaction with the same number.

I'll leave this forum available for any additional queries.