Vatfree.com
Why do I need a customs stamp? / A Customs validation stamp on your receipts proves you have exported the goods back to your home country. This is one of the four requirements to claim back VAT.

The other 3 requirements are:

  1. that you get this customs validation stamp within three (3) months after the purchase date of your receipt
  2. that you live in a country outside the EU
  3. that the receipt meets the tax requirements and exceeds the minimum spent amount.

Customs will check all of these requirements before putting a stamp on your receipts.