Online preorders + time window + text for pickup
Agreed - online preorders + assigning a time window.
Here's one way it could work: during time window, they scan your QR code (automatically checking that you're showing up for the right time slot), hand you a tag, and then text you when your stuff is actually ready to pick up. At pickup line you just give them the tag and they find and hand you your bag. Done and Done.
If there's a delay for some reason, they could text you to let you know there's a delay and when to show up, much as airlines do if your flight is delayed.
If you don't pick it up in time, they could possibly assign you a second-chance pickup time window, but hopefully that would not be necessary.
Another thing they could do is offer optional merch pickup the day before the main conference, with the same system - checkin time windows and texting you when your stuff is (or will be) ready to pick up.