This endpoint can be used to create or update a single POS item.
Don't use this endpoint to upload the entire product structure.
This is for small updates (for example, when a new item gets added to the catalogue at the POS). Do not make multiple requests to this endpoint with the goal of uploading the entire catalogue, upload items in bulk via the Create Items and Groups endpoint.
We will use the POS ID to look up an existing product. If none is found, we try to look for a product with a matching PLU. Only if an item still can't be found, this endpoint creates a new one.
The POS ID takes precedence over the PLU when looking up a product, so this can be used to update a product's PLU code.