This dialog pops up when you log in today, or change channel if you're already logged in.
Note that it does not ask you whether you want the coin on that character, and there is no "end chat" button either. Clicking Next gives you the coin.
If you want the coin on another char, you need to forcibly close the client.
The Update Notes say only one char can receive the daily coin, but says nothing about the shop being one per account.
The in-game dialog says "Only one character per account can participate in the Black Friday event", which seems to imply only one char will be able to use the shop.
(The in-game dialog also looks like the coin is given once in the entire period, like the Power Growth potion.
Contrast and compare
I guess we will find out tomorrow whether that is the case or not).