Duty free shopping at airport?

I went to a shop and bought a pair of shoes, and they told me I was eligible to buy duty-free as a tourist. They scanned my passport at the time of purchase and did not charge me GST at time of purchase.
He did say it was ok for me to use the shoes immediately, but due to language barrier it was difficult to understand the rest of his instructions, he said something about me needing to do something at the airport? Do I need to show my shoes at the airport with the receipt? What about any other goods I purchase?
I did not keep my boarding pass, will that also be an issue and do I need to ask my airline for a new one?
I did not register for duty free shopping during visit-japan-web signup, will that be an issue and do I need to register for a QR code? I thought that was only for people who did not want to show the passport to the shop staff?
It will be far easier for me to show the passport each time rather than do any kind of sign up (which requires scanning my passport) now that I am already in Japan and on-the-move. Especially since I don't intend to buy many more items.

Some guidance appreciated about what I need to do from now on :)