Even if you had a signing device on you wouldn’t you need to broadcast the transaction?
Discussion
The cashier could broadcast it, that’s my question, how?
Like what things are being built that allow for that interaction of customer signs transaction -> cashier broadcasts transaction to take place?
Using fedi-alpha on fedi network, seems to have an offline feature. Have yet to test it out.have been able to send sats to usernames via chat 
Wow, a year ago
Hmm i have never done that so that would be creating a psbt partially signed bitcoin transaction, then signing it and then giving that transaction to the vendor for them to broadcast the transaction when they have internet. I suppose that’s possible Because you can see the transaction info in your wallet before you broadcast it but I would imagine there is a way this can be “double spent” or otherwise cheated as its just a transaction that you can’t confirm becusee you don’t have Internet. I’m curious if this can be done securely. That being said I have seen cool implementations offline lightning ATMs and games