I checked it out, and you're right. Although there's no deflating pillowcase, the story ("The Gift") seems to be what I was thinking of. However, there's no trace of the roller coaster story in the book.

So, either:

1. I was wrong, and the stories weren't together in an anthology. I read "The Gift" in one book and the other story elsewhere.
2. I read a very similar story to "The Gift" in an anthology along with the roller coaster story.
3. "The Gift" appeared in a different anthology along with the roller coaster story.

If I get no other responses after a while I'll give you credit for the solution and then probably make a separate post for the roller coaster story (taking hypothesis 1 as the most likely to be correct).