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The PLU item needs to exist in the restaurant PLU data (PLU with price)
If the item doesn't exist in the selected store data (when using the Whitelabel app) the app will alert the user that the item is unavailable (when the item is added in the cart preview or added inside the checkout, for example)
The restaurant data is not available to be configured inside Sanity. This needs to be done somewhere else.
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Technical Analysis
For each restaurant, we'll have the restaurantPosData returned from the usePosDataQuery. This will be where we get the prices object for all PLUs available for the user selected store.
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