PermaCart
The PermaCart isn't for all store types, it depends on the actual product type within the store, and allows your customers to retain products within a separate "shopping cart" which never changes until they delete an item manually. If they added a product to their PermaCart, and then added to their regular shopping cart to actually purchase and check out, then the product would remain in their PermaCart for the next time they visit the store.

It was essentially created for stores with consumables such as printer inks, paper, food, pet supplies etc... anything that runs out and needs to be purchased again. The idea is that someone would visit your store looking for a particular printer ink (for example) and, rather than place it directly within their shopping cart and check out straight away, they'd also add it to their PermaCart.

The next time they visit your store, the printer ink would still be within their PermaCart, with an option to purchase this item. Assuming they now need to purchase more ink, they would click on the purchase option and check out as normal.... but even though they've checked out, the printer ink would remain within their PermaCart for the next time they visit, ensuring that they never have to search your store for that same product again. Any time they need to purchase more printer ink, they simply return to your store and it's already there on every page waiting for them to purchase it, saving them time trawling through your printer inks looking for that particular ink, and encouraging them to return to your store.

Of course... if they keep coming back to your store to purchase their printer ink, there's also a good chance that something else will catch their eye at the same time!