I have created an invoice. When completed it automatically is marked as paid when it hasn't even been sent to the customer. Is this a setting that can be undone.

Hi service12,

Thanks for your post and welcome to the community! 

To turn off this setting which automatically applies any un-applied payments or credits to open transactions please go the Gear icon > Accounts and Settings > Advanced > Automation and deselect 'Automatically apply credits'.

Thanks :)