how much does it cost to ship a 15.5 gallon keg?

Answer 1

What's the keg made of? What is inside the keg? How much does the filled or empty keg weigh?

An aluminum keg filled with beer weighs more than one filled with water because even "lite" beer has more solids in it than plain 'ol H20.
It also depends on where you want to ship to, which carrier you use, and how fast you want the keg to arrive. Extra heavy packages may have a surcharge.

Check the carrier's restrictions lists, and pack the keg in foam peanuts in a larger box.