Every time I get a little worried about a big player like Amazon having too much control in a market, I think the solution is actually another big player - Google. Not that I want google to compete, but if the knowledge graph gave info about the restaurant including delivery services that it supported, that would make it easier for everyone. The restaurant could list a few services, and you could choose the one you have an account with, or trust to serve you well. This takes the control that exposure a big player like Amazon has and really reduces how effective it is at corralling suppliers.

Now, I don't really care that it's google, but they are the biggest and most likely to be seen. Preferably Bing and Yahoo and DDG would have an equivalent.