Is it cheaper to mail a box or package?

I'm curious, could you elaborate on whether it's more cost-effective to send a box or a package through the mail? I've heard conflicting opinions and I'm wondering if there's a definitive answer to this question. Specifically, I'm interested in understanding the factors that determine the pricing difference, such as the size, weight, and destination of the shipment. Also, are there any discounts or promotions that can make one option more favorable than the other? It would be great if you could provide some insights and clarify this matter for me.