The smart pantry is a modular system and can be adjusted to the different consumers needs. At its base is a small (and cheap) computer (could be a Chip pc or similar).
This base can make use of a different combination of modules to keep an updated list of the commodities in your pantry.
A bar-code reader module will be a convenient way to scan all packaging that arrives from the shop as it is stored in the pantry and also all packaging as it is thrown to the trash. (a mobile phone or tablet may be used as bar-code reader module)
A weighing station module will enable communication through BLE with small weight sensors that can be placed under strategic commodities that will be tracked by weight.
A touch screen module can be added to allow simple manipulation of the current shopping list (adding and removing items as well as a scheduling mechanism). (a mobile phone or tablet may be used as the touch screen module)
The back-end will use machine learning to analyze your past behavior and to try and make "smart" decisions based on this behavior and the information obtained from the different modules.
The combination of these means and other modules that could be added later should make the task of keeping tabs on current pantry levels more manageable and Amazon DRS feasible for these types of products that otherwise wouldn't be.