Price
When it comes to shopping, price is one of the most important restrictive aspects. To ensure you're truly getting what you're paying for, you not only need to confirm the quality of said product, you also need to find out if you're paying the cheapest amount possible.
Nowadays there are numerous price comparison websites out there that are extremely helpful. Even if you're already in a shop, it never hurts to double check the current value of an item and the lowest price it's ever been.