Oh no! Someone got an invoice sent out after payment. The question is where is the checkbox to turn this off/on? As it happens there are several places that would be causing people to get invoices sent to them.
E commerce -> Settings -> Orders
E commerce Setup -> Recipts -> Orders
If the overall settings above are set to send an invoice the below settings will not override This means if you turn the invoice off below and it is on in one of the above settings, an invoice will still go out.