If someone put something in your website’s shopping cart — they did it for a reason. They are actively in the purchase consideration process.
If they leave it behind — there’s a reason for that too. They found a better offer, they decided not to buy, or maybe they just got distracted.
Don’t let them walk away! Follow up, send them a reminder, and stay in touch. Possibly offer a deal if they come back. Do what you can to close the order … they are so, so close. (Most shopping cart software has this sort of feature built in.)