If you can, use a free Priority non-flat rate box and the rate will often be cheaper. It's just for dense things that flat rate makes sense. I think a large flat rate box runs about $18 to ship, I shipped some buildings last week in a larger box for only $11.
when selling on stuff like eBay too. If I know it will fit in this size flat rate box, I know the shipping will be exactly this price. I know in the past, I didn't have things packaged and weighed before I put them on eBay. I didn't bother doing that until the item actually sold. That said, I've... more
I usually weigh first, up the weight on eBay by at least one class (first class is up to 4 oz, then 8, 12, and 16) just in case things run a little heavier once packaged. But I'm not selling casually just slowly...