Design options for a simple AVR (Arduino) based RF communication system
I did a job at 434MHz that used tiny 0.25mW transmitters and a tiny PIC. Battery life was good - over a year because the transmitter consumed virtually nothing for about 1 minute then woke up, transmitted its small payload and went back to sleep (about 50 ms up-time). There were about a hundred transmitters all monitoring the temperature of individual freezers - basically they were "alarms" to warn of impending defrost should one of the freezers become faulty. One central receiver.

Conceptually I don't see a difference - you have a bunch of transmitters all of which can randomly talk to a central receiver. Transmitter current consumption when transmitting was about 30mA (from memory - it was in the early 90s so my memory is not that great on this).

I got the transmitter modules from a UK company called Radiometrix - they were FM 434MHz devices.

Because of the potential for transmit collisions I would also recommend a display that all customers can see that tells them their request is being processed.