It might just depend on staff, cast, and stock of products.
I was there the night the manuscript for Spring Awakening was sold, and that was the one and only.
I was also there over Easter weekend, and it just might have been too much for some shows.
Don't count on being able to get in after the show, some of the donations are hidden far within the theatre, and I doubt they would let someone walk in. Plus, I think it would be difficult to fight the crowd.
So, that was the Drowsy Chaperone. Oh, I love it so much. I know it's not a perfect show...but it does what a musical is supposed to do. It takes you to another world, and it gives you a little tune to carry with you in your head for when you're feeling blue. Ya know?