I think the extract you provided from the statements makes it clear that, just as with a credit card for example, the payment needs to have reached your account by the due date, and that probably means if you pay online you need to pay a couple of days before .
I would be very surprised if they were providing credit to you if you hadn't signed/agreed on line to the credit agreement.
But if you have been a good customer , pay what they ask in good time next time and then write to a senior manager explaining that these charges came as a great surprise and you would like them to refund them as a goodwill gesture, otherwise you may not be able to afford to do business with them in future