The checkout refuses to complete with a message when:

• "You need to login before you can checkout" – not logged in.
• "Error! Your Cart is Empty" – the proposal has no billable lines.
• "Error! Your Total should be greater than zero" – the remaining amount is 0.
• a delivery-zone problem for the event address (the zone message is shown).
• "You need to agree to the terms & conditions!" – Force Agree on cart is on and the box wasn't ticked.
• "Error! You have errors on Event Time Field" – catering items without an event time.
• "You need to select a payment method!" – no method chosen (or an unsupported one) while an amount is due.
• Any gateway error (declined card, etc.) is shown as returned by the processor.

If the payment history already covers the total, opening the checkout shows "After taking a look at your payment history this proposal is considered fully paid, Please consider reloading the proposal's page" and the proposal is set to Fully Paid.