I understand that you need to customise the payment receipt. Let me share something about it.
In QuickBooks Online, we only customise the following:
Invoice
Estimate
Sales receipt
This means that we're unable to personalise or make some changes to the payment receipt template. However, please note that it uses the Standard form for its basic styles such as font and color. Go to the Gear icon, select Custom form styles, and find the Standard or the Default Form Type.