I'm honestly not quite sure - the desk is open most of the day, but the staff contracted by BA at Schiphol are ... not the most willing and helpful people at times.

However if that doesn't work there's a left luggage facility here: https://www.schiphol.nl/en/facilitie...-term-storage/
so either way you can go to the airport, try the checkin and if they are unhelpful use the left luggage facility.