It depends on the facts and the communicative context. If the contractor whose gender is known, then say “he” or “she” as the case may be. If it’s a company, in American English we sometimes use “it”, but we very often use “they”. In British English, I believe they would certainly use “they” to refer to a company (with multiple people working there). Finally, if you really cannot figure it out, you can go ahead and use singular “they” these days (its actually been used for hundreds of years). So, without further context, your safest bet is just to use “they”. Taking a second look at your question, if you’re going to write to the contractor, why don’t you call them “you”?

Usually business letters have greetings written this way: Dear Mr. Smith / Mrs. Smith / Ms Smith / To Whom it May Concern You can get away with not using any personal name by writing a subject line after the names/addresses to make your letter look like this: ABC Company 123 Forest Lane Anytown, USA June 28, 2020 Ahmad Alattar 321 Brookside Street Anytown, Kuwait REMOVAL OF GARDEN ROCK AT THE SMITH RESIDENCE IN ANYTOWN USA (↑↑↑ This would be underlined and centered on the page.↑↑↑) Thank you for taking the time to review my request to proceed with the project to remove all garden rock and other yard waste from the Smith property, located at 1010 Berrywood Road in Anytown, USA. I have e-transferred 50% of the estimate to start the project, and will pay the balance owing upon completion. Thank you again for all of your help regarding this project. If you have any questions, please contact me at 965-000-000.