Why do we like expected surprises? The ambiguity effect says that we try to avoid uncertain outcomes. Still, many products are selling well excactly because they give uncertain outcomes. One example is Kinder Eggs, and many similar candies\toys. Some shops also get rid og their spare products by selling them in 'surprise bags'.

Why do people like buying products with uncertain outcomes, if the ambiguity effect is true?
There is no ambiguity as such with respect to expectations in those examples. When you buy a Kinder Egg, you know up front you won't know what's in there. In other words, it's perfectly unambiguous that you don't know what to expect from a Kinder Egg.