AddProductToCart adds a product to products database.

BuyItemFromCart is the helper function for database interaction for the BuyFromCart function in controllers.Cart.

InstantBuy is the helper function for database interaction for the InstantBuy function in controllers.Cart.

UProductData returns a *mongo.Collections for the product's collection.

RemoveCartItem removes a product from the user's cart.

UserData returns a *mongo.Collections for the user's collection.

Client is a client for connecting to the database.