Put a note on your PDP or lander that the customer will save a certain dollar value if they purchase another item.
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Increase the threshold for free shipping by $3-$5 above your current average order value. Monitor your AOV in Google Analytics to see if it continues to work.

Use related products or complementary items to encourage customers to add more to their cart. You can also offer better or more premium versions of the product they are already purchasing (Upsells).