Translation:I want the most expensive snack in this shop.
It could be duo's mistake...
But, thinking about this question, ichiban is usually translated as 'most', although it literally means 'number one'.
If we use that meaning and translate the sentence to 'number one (most) expensive treats', it is incorrect in english because number one requires a singular noun due to the fact that there's only one number one.
I'm not 100% sure though that the japanese use it to explicitly mean number one, I'd need a native speaker to confirm that...