I think I figured out a simple way to create a glass deposit. I did this for my "Growler" (a big glass jug) items on my site: https://culturekombucha.ca/collections/all
-First, I created a product called "Growler Deposit". I set the price, no tax, and no to tracking qty. I made sure it was active on my sales channels.
-Then, I downloaded the app: "EasyGift: Auto Add to Cart ". (It's $9.99USD/month) I created a new rule and picked Product as my rule type and chose the products that needed a deposit. I then added my "Growler Deposit" as the item to automatically add to the cart. In my case, I selected the options, "Do not apply any discounts" and "Rule works in reverse".
-The next part is to "hide" the "Growler Deposit" from my collections page so customers can't see it on the store. I went to Collections and under Conditions, I did: Product Title: Does not contain: Growler Deposit
I hope this works for you! My next step is to figure out a way to add a credit to a customer's account when they do return the glass.
Just one thing to flag is as you change cart contents the product quantity doesn't change so you could end up with incorrect cart totals. We spent a lot of our energy on fixing that issue for carts. We are still going through the shopify app store review process unfortunately.