We are using version 6.1.5 right now, so this may have been fixed in a later version. But we found an issue when using a coupon with a set amount.
We have PayPal as the only payment option.
When you have a coupon with a value that is greater than the price of the event, the call to PayPal is skipped (correct) - however the invoice shows a negative amount paid and the status of the booking is set to "Pending" rather than "Participation Assured".
For example, we have events with the price set at $30, $40 and $50 and a coupon code that has a price of $40. We set the value of the coupon as a fixed amount to allow the customers to choose either the $30 or $40 events and not have to pay anything. However, we did not want the $50 event to be free with the coupon.
If the customer picks either the $40 or $50 events, the process works correctly. There are no issues.
However, if the coupon is valued more than the event, the booking is placed in pending status, I believe, because the balance is not zeroed out. I believe the code just needs to be updated to set the balance to zero when the coupon value is greater than the event cost. Currently, the balance is showing as a negative value (-10.00)
Here is an example of an invoice for this issue: