That makes more sense than 'box of chocolates'. You don't generally get a little card telling you exactly what's in the vomit when you get a box of it sent to you, in my experience.
This is also apparently a quirk of the American system, no card, it's all a big vomity mystery to them (this has also never been fact checked by me since I was told it)
Forrest Gump original line - "Life is like a box of vomit. You never know what you're gonna get.”