The Basics

Before starting, sign-in to your Amazon account (create it if you haven't got one). Amazon's requirements for free shipping to Singapore are stated here. Basically, must buy a minimum of $125 worth of eligible items.

So how to find eligible items? 

1. Start by searching with the search bar then ticking this checkbox:

2. Next, filter the search results to remove most of the ineligible items by doing this:

3.  With the search results filtered as above, it will now be more efficient to browse individual items listed in the results as most of the items listed will be eligible. When you click to enter each item's page, you can further check for these criteria to be a bit more certain about whether the item is eligible, but this is not necessary because it may not be 100% accurate. So you can simply continue shopping to add things in your cart until you are ready to do the next step (STEP 4). 

4. With at least $125 of eligible item(s) in the cart, the final step to is to verify the eligibility of the entire cart by doing a partial checkout. In the checkout process, after entering/selecting a S'pore shipping address, this page will be shown. Here, the free shipping option will be available if all items in the cart are eligible:

If the free shipping option is not shown, it means that one or more items in the cart are not eligible. Follow step 3 and carefully check again to see if each item meets the eligibility criteria. 

Tip: If there are many items in the cart to check, the quicker way is to leave just one item in the cart by shifting all other items into the "Saved for later" section. Then by increasing the quantity of that item until the cart value becomes $125 or more, a partial checkout will quickly ascertain if that item is eligible for free shipping to S'pore.