Promotion Letter Formation

I am Joshua Sundar, working as Salesman in Jarir Book Store of Saudi Arabia. I am working since 4 years here as salesman and my performance was good . Last year i was able to achieve my assigned sales budget and this year also until now going well. i was a Post Graduate in Business Administration with over of 9 years of selling expereince. Now i would like to request my management for promoting me to Purchasing Department. Can you please help me in making an appropriate letter to my Big Boss . Thanking you (Joshua Sundar)