Thrift Shop DOG mystery bag!

mystery bag (plural mystery bags)

1. A bag whose contents are unknown. 

   Synonym: (informal) blind bag

2. A bagged package, randomly picked, which contains surprises (e.g., mystery prizes). 

   Synonym: grab bag

Our gift bags are available in either cat-themed merchandise or dog-themed merchandise

The key element is surprise.  Surprise and whimsy.  Two.  Two key elements, whimsy and surprise...and good value.  Three key elements.  Three.  Surprise, whimsy and good value.  And all proceeds go to the Winnipeg Humane Society.  Four.  I'll start again...