Where should I send the 20 unit challenge samples I'm ordering from my supplier?

You can send the 20 units directly to your house.

You will send them to your house. This is because you will need to take pictures of your product and also ensure they are labelled correctly. You will also need to apply the Amazon shipping labels to the master carton. 

Remember: If you live in the USA, you will send them to Amazon.com
If you live in Canada, you will send them to Amazon.ca

If you live anywhere else, send them to your country's Amazon, OR, send 19 units to Amazon.com and then 1 unit to yourself (so you can still take pictures!).