After seriously lengthy/much negotated negotiation with the customer, they are unwilling (I think you mean willing?) to accpeted accept the goods by latest 20 May at the latest. Please keep any your eyes on the schedule - no further delay will be accepted.
I'm a little confused. It would make more sense if the customer had now agreed (be willing not unwilling) to accept the goods by a certain date. If they are still unwilling, then what was the point of the negotiation? It makes sense if they have now decided that they will accept the goods on or before 20th May, but absolutely no later.