In the US last mile is super hard because of the super high wages. The only way to work around it is volume per delivery person.

If you do 2 deliveries per hour (like Uber Eats / door dash), you pay essentially $5/order (assuming a super low us wage of $10/hour and no equipment cost/ gas).

So no in the US, Amazon is not threatened by such delivery services.

Now if you go to China, the equation flips. Which is why Amazon failed completely.