I'm glad to see someone else thought of inventory management for this jam theme! I really like your take on it. Combining items is fun and responsive. I agree with others that number keys would improve the experience.
Part of me wishes you could overwrite a slot's contents if you don't have a matching item, but I do like the strategy of not locking yourself into one type of item. Maybe you could overwrite but at a penalty to your score?
Great job!