For a webshop i am looking for an extension for the shipping costs
$0-50 -> $5
$50-100 -> $2.5
$100> -> 0
You can configure that special pricing using the "Table Rate" shipping method that is included in Magento out-of-the-box.
The configuration would be "Price vs Destination", and you should apply the shipping cost to "all destinations" depending on the price.
Here you have more information about it: