How has a customer managed to pay off a pre-order early?

By default when any customer purchases anything on Shopify, they’re taken to a thank you page.

That page usually just displays an order summary + shipping details. However, as per below there’s a blue “Pay now” notice.

Customers can re-visit this page and choose to click “Pay now” and pay off their order.

This is controlled by Shopify and currently can not be disabled. The good news is, if the customer actively chose to do this, they should be fully aware of the situation.