Do you mean emailing them or a stamped letter to them? I wouldn't know the email address of the CEO.
Most times if you root around on Google you will find what you want. I'm inclined to use registered post which costs around €9 but you don't have to go that far. Address appears to be here.
You could when you are in the store on Friday forewarn them of your intention to write to the CEO and you will not be letting this go.