For some reason I have never walked into a shop and went to purchase something only to see an ad on that something (in the shop!) that says I can get it cheaper if I buy direct from the manufacturer. I may see ads elsewhere that say that, but definitely not in the store I just walked into.
Except that what Spotify is looking for would be more like someone walking into a shop, picking up a product and being able to see copy promoting an alternative and possibly cheaper way to buy that product through the company's website. HP, for example, promotes its Instant Ink subscription and website on packaging for its products found in retail stores. Apple is preventing something similar with apps.