If you are going to spend time writing a letter about how rude an employee was to you, then take the time to do it in a way that your complaint is heard and acted upon.
That means finding out to whom it should be addressed, is it the owner, the manager or the supervisor?
Make sure you include the time and place the incident occurred and any other details about the actual employee, like their name and ID number if possible.
When you write about the incident, avoid the urge just to complain and say mean things about the people and the company. If you do write a letter in that style the person reading it may assume you were a demanding customer and that the employee may have had reason to treat you the way they did because you were unreasonable.
Here is an example.
Attention (Company/Business Name)
To whom it may concern / To the Supervisor (name) / To the Manager (name),
My name is Jane Smith, and I would like to lodge a formal complaint regarding the rude treatment I received by one of your employees on the 2/5/2017 at your store.
It occurred approximately at 1 pm as I was at the Checkout number 5. The employee working the cashier was wearing the name badge, Lana Rogers, ID number 12345.
After having scanned all of my items, she requested the payment. I replied that I had already given her my Bankcard before scanning the products.
Miss Lana said that I had not and insisted I pay as I was holding up the line, I repeated to her that I had already given her my card and kindly asked her to search the workspace for it. She rudely told me to stop telling her what to do and that not only had I not given her my card, but that if I did not pay promptly, she would call security and have me removed as I was holding up the line and creating a disturbance.
At this stage, she had begun to raise her voice to me, however, the customer behind me kindly came to my defense and pointed to the card which had been placed by Miss Lana on her receipt machine.
Miss Lana then swore quietly and processed the payment without another word to me.
I have been a long time customer at your store and was most upset by these events. Since it happened, I have been doing my shopping at another local grocery store as I was most embarrassed and did not want to experience this ever again.
I hope that this letter explains my feelings and disappointment with what has occurred, I would not like this to happen to me or any other customer and hope that corrective measures are taken swiftly.