I've recently started getting into Google Merchant. I see an option to add checkout to the listings which requires a link to the cart including an item ID.

Here's their example: https://www.mystore.com/checkout?item_id={id}

Is there a version of this I can enter for our store? A simple link to cart.asp won't work

Hi David, I don't think that's going to work.
You should consider a product feed and the listings would send customers to products.
There was some old code some years back that it was possible to link to the cart with a product added, but it was more elaborate than using a querystring like your example.

Mike at www.dlss.ca has a feed generator.
Google has also begun adding products into their shopping via the Schema data, which the products already have some of that covered, and Mike also has a nice tool for that too.

Thanks,
David