React, make this component re-render

This started as my Product Landing Page for freeCodeCamp, but I just kept going. I’m working on a shopping cart.

In the view cart page (right now there is only a link to this page in the footer and you need to add products before you see my problem) I’m trying to make an add item button. It does add an item to the cart but it does not make it re-render. If you add an item, go to another page, and then come back it has the correct (new) number of items. How can I make it re-render directly when the button is clicked.

Thanks in advance for any help!