I would send them a message like this:

I apologize that you can not locate your package. Please check with neighbors in case they received your package by mistake. We ship all of our orders with Amazon's Buy shipping Policy where you will be protected if you do not locate your package. Please follow the below steps to make a claim:

You may be eligible to request a refund under the A-to-z Guarantee if “You have not received your package and three days have passed since the maximum estimated delivery date or the tracking shows a delivery confirmation, whichever is sooner. Note: Amazon also requires you to wait 48 hours after contacting the seller before you are eligible to request a refund.”

Requesting an A-to-z Guarantee Refund

To request a refund on an eligible order:

Using a LAPTOP or DESKTOP device, Go to Your Orders.

Locate your order in the list and click Problem with order.

Select “Package didn’t arrive” from the list.

Select Request refund.

Enter your comments in the text box.

Select Submit

To request a refund on an eligible order using a MOBILE device or CELL PHONE:

Call 8882804331 or use Amazon’s mobile app to connect to Customer Support 2.

Report that you “did not receive” your order.

Request a refund.

Note: Refund requests can take a week or more to investigate. Amazon will notify you of their decision via the email address associated with your account. To view the status of your request, locate your order in the list on the Your Orders page and select Problem with order.

To receive the maximum refund allowed you need to select “Package didn’t arrive”