On-demand music streaming app Spotify has now formally added one of the three music retail affiliates it told us about earlier this month. Listeners can right-click track names and pick “Buy From 7Digital.com”, letting them purchase corresponding 320Kbps MP3s and some FLAC files from the six million track-strong service. Later, users will also get to buy whole playlists they create inside Spotify.

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With Spotify gaining a loyal audience, it’s a good deal for CEO Ben Drury’s Shoreditch, London-based 7Digital which, by virtue of its API, also this month inked a similar deal to be a retail affiliate inside Mozilla-based media player Songbird and which also powers some downloads for sites including Last.fm. For Spotify, it’s the start of an a la carte downloads model that could supplant its core ad-supported, per-per-day and monthly subscription models. The commercial split is unclear. As is one other thing: since Spotify’s model is all about streaming from the cloud, who will really buy individual tracks through it?