I just wanted to share this, with the hope that it will help others. When I first started Kimbo's Cookies, I wanted to provide shipping at the lowest possible cost to my customers. The problem was that, on Etsy, I had to list a shipping price, so I would estimate the weight of an order and low-ball the cost. I lost over $650 in shipping fees that first year. I now ship everything flat rate. I have boxes from BRP that fit one dozen and two dozen nicely. A medium flat rate box will hold one dozen (with lots of room for bubble wrap) and a large flat rate box holds the larger order.
I was so scared of losing business because of high shipping costs, that I burned myself. So my tip is, use flat rate boxes. It makes invoicing and/or Etsy listing a breeze.