My invoices have started being sent out with "PAID" on them - I've not changed any settings so don't know why but its now causing problems with customers not paying

Hi stewart7,

Thanks for joining us here on the Community 👋

The most likely reason for this is that any un-applied credits are automatically applying to the invoice. 

Please check if you have this setting enabled by going to the Cog Wheel > Account & Settings > Advanced > Automation > Automatically apply credits. You can disable the setting here if you no longer wish to use this. 

For the invoices which have been marked as paid, you can un-link the credit payment by opening the invoice and selecting '1 Payment made' > Select the date to open. On the following screen, un-tick the credit which appears and change the payment amount to zero. 

Please get back to us below if this does not resolve the issue. 

Thanks