If it is in your contract, just point out the mistake and ask to have it fixed before closing. They can figure how to do so. The sooner you bring it to his/her attention the better.

It's not too late, and it's not too much trouble. If it was something they were supposed to do, but didn't, definitely say something now, before closing.

Not too late. If you paid for it, you should get it. Make them come back and do the work.

It may or may not be easy to accomplish at this stage. If it is included in your signed contract, it is not your problem to figure out how difficult it may be. It’s your builders issue.

He may offer to give you a credit instead of doing it at this time. He also might want to credit you the “open wall” amount, rather than “remodel with walls and paint in place” amount. Don’t let $300 vs $75 stop you from closing. But do be sure that you either get a proper credit, or another no charge upgrade equivalent.

As stated above, you can ask for - and expect - it to be remedied. There is a second option: ask for a refund for the missing item. Many builders would prefer to offer a decent discount rather than have to schedule a couple of their workers to come in and finish the work and then do the patch/repairs to the finished surfaces.

You are the only one who can decide which is more important to you - the feature or the money. I would do the walk through, point out the deficiency and then ask for a quote for both options. A "time quote" for the work to get done and a "refund" quote for allowing it to stand as-is.

At that point, you can decide if the refund is good enough compensation for the loss of the contractual item. Personally, I'm all for a bit of money back into the bank at the end of a build....but that is JUST ME. This is your house, you need to be happy/satisfied with what you get.