Hi There May.
I have the same problem. We have a 20 seater restaurant and sell reservations over shopify.
There might be only 2 seats open, but customers are able to add lets say 4 seats to their cart. When they check out there is a small warning and the quantity goes back to two automatically. Problem is people don't read. They just check out and assume they have a reservation for 4 guests. They then show up very annoyed at the restaurant.
I have tried the following fix but it doesn't make any difference.
I also have a box with the available amount of seats right next to the quantity selector on the cart page, but still get customers "over booking".
If i can have a pop up message saying that you added more items to the cart than available, like the link claims to do, that would be great.
Would appreciate your help.