Some invoices I have sent are showing as paid straight away without me doing anything!

Hello and welcome to the Community, info-ejeventmedi

We'll be happy to share some insight as to why some invoices are being auto-marked as paid. 

This happens when the setting to automatically apply credits is enabled. This setting applies any open credits to the next invoice you create for the same customer.

You can manage this setting by going to the Account & settings > Advanced > Automation > Toggle the slider to the left (Take me there).

If you need any further support on managing invoices in QuickBooks Online, please get back in touch below. Have a great day!