I bought a food scale that weighs up to four pounds in 0.1 ounce increments. It seems to be accurate within 0.1 ounce - so it's really worth while to use for guesstimating a shipping cost.
It's also worth the time (in many cases) to check usps and ups and fedex to compare the costs.
I usually use usps because it's the closest drop-off site to my home.
This is a good opportunity to learn a bit more about different professions.
There are all those costs we don't see -