If a customer pays me by credit card using Sum Up how do i register payment and show the customer has paid in full even though fees have been deducted?

Hello @info1583,

Normally, recording the payment and closing the related invoice is based on how the payment solution you used processed them.

With this, I'd recommend contacting the Technical Support of Sum Up. With their tools, one of their specialists can explain how the payment solution integration works. 

Additionally, I've also included this reference for a compilation of articles you can use while working with us: Sales and Customer for QuickBooks Online.

If you have any other questions, please let me know in the comments below. I'll be here to lend a hand.