Invoice stamped by a non-EU country's customs office is invalid. The European country that stamps your invoice must be part of EU. For example, if Switzerland is your last destination in Europe, please visit the last customs office in EU territory before reaching Switzerland.
For example, if the customer is going to Switzerland by train, he or she need to step off the train in the last station before arriving to Switzerland, and get the invoice stamped by the customs office inside the station.
List of european countries that are NOT part of the EU (invoices stamped by these countries are INVALID):
Principauté de Monaco