I just like to get the attractive promotion letter, which I will send to different clients to ask them choose my Agency for their all kind of Air Ticketing and get Airlines promotional packages on very low cost.

Where is this at ? Different countries as different ideas. But you hire a professional advertising company and figure a campaign

People don't read letters.
They want prices. Print a glossy brochure or postcard and plan on wasting a lot of money on postage, with maybe a 2% response rate, if you are lucky.
A website with good keyword search his how it's done now.