My Zen Cart plugin Save for Later has just been updated for PHP 7.3+.
Save for Later allows your customers to set aside items that are in their cart so they can purchase a smaller number of items, but still come back to the items they have already configured (if there are attributes) and purchase them later with just one click to add them to the cart. Like a number of my plugins, it was modeled after an Amazon feature of the same name.
In the image below, clicking Checkout would result in purchasing 2 of the Value model of G200 MMS. After Move to Cart was pressed, 7 of the Premium model would be added to the cart.