New house and constructor wants to charge me fees 1 week before the closing date.. help??

Hi all, I bought a new house 5 months and I want your opinion about a fee that the constructor wants to charge me last minute.

I specified I want my stairs black and my hardwood floor grey a long time ago. It was all good until now. They called me to say that normally for the last step of the stairs they put hardwood floor because the last step is bigger and they are normally putting the same hardwood floor that they put in the house. So it's either that the last step will be grey or I pay 350$ so they put black wood.. I would understand if they would told me that at the beggining, but they are telling me this 1 week before the closing date lol...

Can they do that? What should I do?