I am your potential credit card customer for the past 2 years.
Well this is not really necessary for your letter. Your letter is not intended for impressing him about your creditworthiness or anything. You need to lodge a complaint that you lost your credit card. Follow these steps when you attend to this task;
1. Identify the type of letter you are being asked to write.
2. Open and close the letter correctly (Formal - Dear Sir, Semi formal - Dear Mr Silva, Informal - Dear Tom) (Formal - Yours sincerely, Semi Formal - Your sincerely, Informal - Best Regards)
3. Open a formal and semi-formal letter with a formal sentence
4. Open an informal letter with a general, friendly paragraph
5. Identify the main purpose of the letter.