Why does it cost $20 to ship two boxes of ammunition?

Simple, on light packages over half the cost of shipping is fees. Those fees are the same if you ship 5lbs or 60lbs and that is what drives the cost. For example, it can cost $18 to ship a small package weighing 5lbs and $28 to ship a large package weighing 50lbs to the same zipcode.

The difference is that the $10 in fees (residential fee, DASI fee, fuel surcharge, declared value fee, pick-up fee, and signature fee) don't increase for the larger package, just the base rate. The best rate will always come from maximizing your order volume to minimize the percentage of cost that comes from fees.

For a detailed example please see this page.