Free Shipping Threshold
Amazon offer free shipping on many purchases that go over $35. You can check the eligible items on "Free Shipping" list. It is not necessary tht you buy single item of $35, you can add mutiple items.
If you join Amazon prime then it will give you one perk that is free 2-day shipping. You can see the prime icon next to the items. Amazon Prime has a cost of $99/year but you can sign up for a one month free trial.
You can select "Standard Shipping" during checkout to get the lowest shipping price for the items you want to buy.