To the best of my knowledge Apple does not use Apple Pay in any of it’s Apple Retail Stores. iStore is an independent merchant and chooses its own business practices and services. If iStore accepts Apple Pay and their service was down, it’s most likely a merchant POS issue or a network issue. Apple shows no recent outages on Apple Pay.
Apple System Status
https://www.apple.com/support/systemstatus
In my experience from past visits, many merchants in the Netherlands do not accept credit cards, preferring debit cards. Again, Apple can’t interfere with independent merchants and their business practices.