Have you struggled to try to figure out a successful formula to use to calculate shipping without having to actually pack the item so you’ll have an idea of what to charge a buyer?
Have you found that even the best shipping calculator doesn’t get it right or the packing materials to keep the item safe weighs more than you had thought they would?
Over the years, I have developed a very simple formula that has been extremely successful. Very rarely, it will undercharge slightly and occasionally it 
will overcharge, but it usually gets within a few cents of the actual domestic shipping for the U.S. Postal Service.
If there is an overcharge of more than a couple of dollars, I do refund the buyer. Most don’t notice the refund, but it feels better to me to give it.
So the formula is this:
Take the weight of the item, round upward to the nearest pound, and then multiply by 2.
The only thing that I have had this formula fail on when using the U.S. Post Office, is very large packages that the post office uses dimensional weight rates. Also, it will overcharge for media mail pieces, which I just simply list the weight rounded up to the nearest pound to get accurate shipping to put in the listing of the items.
Photo by chuttersnap