Keep in mind that ‘hotcake’ isn’t a modern word and that pancakes haven’t been known for selling well for a century. It is understood, but weak.

This is a mixed metaphor. Hotcakes never flew off the shelves, because they are not placed on shelves for sale; they are made-to-order.

Both idioms are frequently used, but they're usually not in the Simple Present tense since it expresses things that are generally true or happen all the time. The popularity of a product is always subject to change. Also, you shouldn't use them together in the same sentence. Use one or the other to avoid mixing your metaphors. They're flying off the shelves. They've been selling like hotcakes. They were flying off the shelves for a while, but then suddenly you couldn't even give them away.