For this to work well we hope you use url signals for you filled shopping card. If you have something in URL that refers to the shopping card its easy to pickup that URL. So for this example lets say we have /&add_card=12312&cat=5&order_idem=993 in the URL. The add_card part is the signal the user added something to the card (item 12312) from category 5 (books).

The payment page confirmationpage also has also has the order_item number in the URL so we can confirm if that order was paid.

Step 1: Create segment - Items in Card but unpaid

Creat a segment just like the below image

Step 2: Show free shipping

Create an action to place content from your stock that says free shipping on every page, if the users is in this segment the next time he comes to the site.