A lot of times of it’s a very small, light item, there are contracts that allow for expedited shipping through government channels which might reduce cost which is why the faster option is less. Probably if it were a bigger item, the faster option would be either unavailable or way more expensive and the slower would be about the same price. Similar to how in the US the first class mail is very cheap.

Also, it’s probably so expensive because expediting the customs process is mostly there just for rich people to skip the line kind of thing like in many other areas of society. There’s almost always a backlog in customs combined with “antiterrorism” things like radiation detection, xray, pathogen detection, poison detection, etc., that is often done at random in large batches, but with expedited they often have to test every piece rather than rooms full of stuff. And routing is similar. They also can’t wait to add your package to a larger batch of things when moving between various points along the way, so a lot of things are moved in smaller batches or direct courier. All of that makes it very expensive to move something very fast, no matter how small.