Shipping Fee for Non-Free-Shipping Items/Parts
First and foremost, if your cart product value is over USD89.90(such as a main product), 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 especially parts
2. For example you want to get a pack of screw. Bottom of description shows shipping weight of 0.025kg.
3. You added that to the cart, and it shows the total cart weight as 0.025kg as well(obviously) since that's the only item you added.
4. You click Checkout, the Shipping fee is US$9.50, seems expensive for just a pack of screws weighing 0.025kg. This is because it falls into the 0-0.5kg weight bracket which is US$9.50.
5. You can get more value out of the $9.50 shipping fee by adding more items to your cart till it hits about 0.499kg, as shown below.
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.