The reason it will not allow you to check-in is that you have selected to add a bag. If you choose not to select to add a bag during booking process it will in fact cost you approx double the online bag fee plus the same for check-in fee.
I was pretty sure that there were no APIS charge if you did it online during booking only if you turned up at the airport as you would have to pay a check-in fee to have it done.
There is also definetly a opt out option for selected seating but it may not be clear.