Given a travel related situation, and given a specific question about the situation, summarize the question into a title

Every time you shop at airports the shop personnel ask to see your boarding pass even if there's no discount on the merchandise due to the lack of VAT. Given that the GDPR is now in force starting from 25.05.2018, can I refuse to show my boarding pass at EU airports, citing privacy concerns? Or can I at least demand an explanation from the shop keeper of how my personal data is stored?

Does the GDPR entitle me to refuse to show my boarding pass at EU airport shops?