With this on, your merch shows up right inside the registration flow. After an attendee picks their ticket and fills in their details, they see an "Add merch — pick up at the event" section. Anything they add rides along into the same checkout as their ticket — one payment, one card entry, one receipt.
It's the highest-intent moment you'll ever get: someone who just decided to come is the most likely person to buy a shirt. Catching that impulse at registration beats hoping they open the app again later.