Hey there, @Lesley-Robson.
Are you trying to link your mileage data to the invoice you've made to your client? If so, this option is unavailable in QuickBooks Self-Employed.
However, you can manually create an invoice and use your mileage data to record it. You can also add the details on the description field so that you can classify it. Let me show you how:
- Take note of your business miles expense.
- Go to the Invoices menu and select Create invoice.
- Choose your client's name in the Customer drop-down list and enter your mileage data on the fields.
- Click Save.
In case you forget to take your device with you, you can manually add your trips in QuickBooks.
I'm just a reply away if you have other questions with the QuickBooks Self-Employed. Just add the details of your concerns in this thread, and I'll help you out.