People resent paying for shipping, especially for low priced items.
Have a bundle package for many of your products, so when they pick one of your products in your e-commerce store, show them that if they buy the bundled package (more of the same item, related items, etc.), they’ll get free shipping.
The best time to sell more is when your client just bought something from you.
After completing a low price purchase (that just qualifies them as a buyer), don’t leave the high priced item to an online upsell option. Instead call them immediately, ask them a few qualifying questions and do a whole upsell process (that you can control much better) over the phone (compared to an online simple buy option.)