Here is the message I get printed from the Customs on the shipment :
"PLEASE REMEMBER TO ENCLOSE 5 COPIES OF THE INVOICE
- For all good samples and shipments destinated for outside the European Union.
- Shipments without invoice may be held in customs and could cause problems for your consignee."

So to insure your packages arrival over Europe, better to include the required 5 invoices copies (they explained that Paypal payment e-mail is valid as invoice copy).