Why does my invoices mark as paid before have been sent

Hello Callumsmith336, 

Welcome to the Community page, 

If it is automatically marking the invoices as paid you more than likely have this turned on in settings, if you don’t wish for the system to do this automatically for you if it just a case of going into the settings and turning it off so it does not continue to happen. 

Cog wheel>account and settings> advanced> automation> turn off automatically apply bill payments and automatically apply credits to off if you don’t want it to do this for you> Save > done