[sytse]

You still have to pick an entity to bill from. If that is a foreign entity the US customers need a special form from you to do business with you https://www.irs.gov/uac/form-w-8ben-certificate-of-foreign-s...

And having a postal box office doesn't solve cashing a foreign check in the Netherlands.

So as a first step I recommend incorporating a US entity (Inc. in Delaware), setting up the PO box (we used Earth Class Mail), and getting a bank account (heard good things about first republic bank).