I have a feeling that wouldn't be legal.
When I had to change price tickets, from memory, the lower prices went into effect that morning, but any price increases wouldn't go into effect until the next day?
The customer always receives the lower price, and I didn't get screamed at when I hopped back on the cash register.
Yep, if it connects to an external audio device first - sometimes it does connect but then disconnect, but usually it works without issue!