How to send notification via bluetooth device
This could be done with BLE beacons. These are low power BLE devices that run in (unidirectional) advertising mode and thus can transmit (advertising) messages (called attributes) which are not crypto-secured and readable by every BLE enabled mobile device without pairing required. 

But on the mobile side, you would need to write an App that detects these beacons and you would need your customer to activate this App when they are in your shop.