If you just want a guess, my rule of thumb is $20 per kg.
tldr; Use the shipping calculator, just follow along, it'll tell you exactly what you personally need to know and you don't have to keep reading guides.
Long version: A more precise answer depends on the size of the parcel, your country, the weight, the volume, the items, the shipping service and many other factors. The agent is arguably least important factor.
Luckily, the shipping calculator handles most of these parameters, is able to tell you the exact price for any country, any agent and any shipping service!