Limit the number of people that can purchase a specific upgrade and remove that upgrade for sale once people purchase all items.
In your https://upgrade.chat/upgrades go to the advanced section and set your limited inventory for the products you want to be limited to a certain number of people. Every time someone purchases, the inventory count will decrease until it hits zero. When there is no inventory, the product will disappear from the checkout page and people will not be able to purchase it unless you save the product again with a new inventory count above one.
When there is no inventory, the product will disappear from the checkout page.