Hello there, @Ben1991.
I want to make sure you're able to assign the right category of the payment from your client.
In QuickBooks Self-Employed, it's important to categorise the transactions correctly. You can use this article for guidance: Categories and how to categorise transactions in QuickBooks. Also, if you want, you can consult a tax advisor. They can share expert advice on whether you should categorised it as business income and pay tax on it. This is to make sure everything is accurate.
You can also check here for addition information: Custom categories in QuickBooks Self-Employed.
You can always go back here and add another post if you need assistance with QuickBooks Self-Employed.