First and foremost, if your cart product value is over USD89.90(such as a KM), the non-free shipping "parts" you added to the cart will be free shipping. This except main item that requires shipping fee.
1. Below is the example shipping fee chart to US
* this does not applied to item with Free Shipping tag, only items without Free Shipping tag such as parts
2. For example you want to get just a 130 motor gearbox. Bottom of description shows shipping weight of 0.035kg.
3. You added to cart, and it shows the total cart weight as 0.035kg as well(obviously) since that's the only item you added.
4. You click Checkout, the Shipping fee is USD9.50, seems expensive for just a motor and gearbox.
5. As you can see screenshot above, the cart shipping weight is 0.035kg, so it falls into the 0 to 0.5kg shipping bracket.
You can get more value out of the $9.50 by adding more items to your cart till it hits about 0.499kg.
6. Now with more items in your cart, but your Shipping fee remains the same $9.50. Now you feel like you have gotten a steal!
* Note : However if your cart has item with Free Shipping tag, it will only display "Shipping" instead of which bracket it is.
Summary : for items without Free Shipping tag, shipping fee charged by every 500gram increment, add till it's before the next bracket, to maximize the fee.