Packing station scans in a fulfillment warehouse

Your company runs a warehouse that packs and ships orders for 200 online shops. When a packer seals a parcel, they scan it at one of 80 packing stations. Each scan must reach 4 systems. The label service books a carrier and prints the shipping label at the station. The email service tells the shopper their order is packed. Billing charges the shop $1.20 a parcel, and a dashboard shows each shift's progress.

Today the station software calls the 4 systems in turn and waits for each. Last week billing was down for 40 minutes. Packing stopped at every station, because each station waited for billing before it asked for a label.

The warehouse packs up to 2,000 parcels an hour. A label must print within 3 seconds of the scan. Billing must charge each parcel once and only once. Next month a fifth system, pickup planning for the carriers, also needs every scan.

What does a packing station do when a parcel is scanned, and how does each system get the scan?