Maybe the workflow could be reversed. First let the workers put the product wherever they feel like; then get the information into the application. From user perspective, the easiest way would be to take a photo of the shelf using their smartphone. (Either the worker is required to take the photo after putting a product on the shelf, or you would have another worker periodically walking through the shop, making photos. I assume the products do not change their places often.) The picture would be analyzed, possibly additional questions asked. The image recognition could be helped by a database that would know which products are typically placed next to which ones.

Okay, I assume the client's next question would be: "Why does it take 10 years and 10 million dollars?"