Pre-ordering merchandise could let fans reserve a shirt or record and collect it at a specified show. The stock available online must match what the band can take to that event.
Orders need a collection location, identification method and clear deadline. Staff should record handover so the same order cannot be collected twice. A customer buying several sizes needs accurate item-level records, not just a single tick against their name.
Start with one gig and a small range. Define what happens to uncollected orders or purchases when the show is cancelled. Test a stock correction and a partial collection. The useful connection is between an online sale and a real handover, with a clear process for exceptions.
Related reading: ListnIN: How a Live Music Streaming App Connects Artists and Fans.