How to Use Alexa for Amazon purchases

You can make Amazon purchases with your Echo device, merely by telling Alexa what you want. This is a cool feature, but it's hard to find good documentation on how it works. Here's my experience from a few tests.

This feature only applies to reorders -- purchasing again something you have ordered in the past -- i.e. the item can be found in your Amazon Order History.
If you try to order something new this is what happens:

Me: "Alexa, reorder Purple Shoes"
Alexa: "I didn’t find that in your past orders, so I've added Purple Shoes to your shopping list."

First test

To test a valid reorder, I decided to order again this shoehorn:

Me: "Alexa, reorder a Long Metal Shoe Horn."
Alexa: "Long metal shoe horn - the order total is $13.99."
Alexa: "Should I order it?"
Me: "Yes"
Alexa: "OK, order placed. See the Alexa app for more details."

The Alexa app then shows the result of the order, and it will also be in your Amazon order history looking like any other normal order.

Second test

To test a slightly more complicated request, I asked for a reorder of contact lens solution. First, I didn't use the complete product title, only a few words from the title. Secondly, I had multiple orders of this item in my past order history.

Here's the item:

Here's how she handled it:

Me: "Alexa, reorder Optimum Cleaning Solution."
Alexa: "I found several matching items in your order history."
Alexa: "First matching item: order total is $12.24. Should I order it?"
Me: "Yes"
Alexa: "Order placed."

I found this to be a practical, useful Echo function, albeit with the reorder limitation. And if you know what you want, it's quick and easy.