Eventually people will say "Alexa buy my groceries"
Alexa will buy from the highest bidder
But for what it's worth, I think there's a conflict of interest here. No one is going to believe that Amazon won't prioritize Fresh or Whole Foods for these orders - even if they don't.
No reason why Amazon can't do something similar. They only need it to be "good enough" that people start getting used to not always needing to ask for it to list what is in the upcoming order.
If I tell Alexa to order ketchup, who knows what brand I'll get or what size? It's possible I might end up with banana ketchup or even mayonnaise given how poorly Amazon's search functions work.