Your proposed letter was well-written enough to get your point across: think succinct; think "what will corporate (Austin, TX, no less) take as a soundbyte from my letter?"
I spent money here every week. I will now spend money at your competitors, not at your store, and influence everyone I come into contact with to do the same. Furthermore, here is a copy of the letter I will send to your wholesalers (local merchants, farmers, etc. - regular people who are likely pro-individual gun ownership and use rights), suppliers, etc. I spent X dollars per week at your business over the last x years. I prefer to spend myh money at your store, but not at the cost of my community's safety, which you put at risk with an advertised no-gun (criminal empowerment) zone. If you retract your policy, I will happily come back and encourage all involved parties to do likewise. Thank you for considering my message.
Madness is like gravity; all it needs is a little push.