When I enter expenses I assign them to a customer - how do I run a report to show what materials have been used for each customer

Good day, @bricksnmortarltd.

You can pull up the Purchases by Product/Service Detail and filter it to show only the expense transactions and the products used for every customer/project. Here’s how:

1. From the Reports menu, select the Standard tab.
2. Type in Purchases by Product/Service Detail in the search box then hit Enter.
3. Set the right Report period and click Run report.
4. Pick the small Gear icon, then mark the boxes next to Transaction Type, Customer, and Product/Service.
5. Click the Customise button.
6. Expand Filter, then select Transaction Type.
7. Choose Expense, and hit Run report.

Check out this article for more details: Customise your reports in QuickBooks Online.

The report should look like the image attached below:

As an additional reference, see Run reports. 

With this information, the report will give the important details, but if you have any questions please leave a comment below. I’m more than happy to help. Take care!