DEAR HARRIETTE: I went to Ariana Grande’s concert at the Barclays Center in Brooklyn. I had an amazing time at the concert, especially since she’s my favorite singer of all time. As a result of my excitement, I felt the need to purchase a $50 T-shirt. When leaving the concert, I made it all the way out of the arena before realizing I left the shirt I just bought at my seat. I ran back into the bui…