"reservation letter" is correct and usable in written English.
You would use it when referring to a letter that confirms a reservation that has been made. Example Sentence: I received the reservation letter confirming my flight from London to Rome.
Reservation acknowledgement letters sent via email or fax within 24 hours.
Despite the European reservations, the Greek letter entailed a climbdown on earlier Tsipras pledges.
And gathering bills, proof of employment, hotel reservations, plus a letter from the hotel to the consulate is pretty dreary and frequently impossible to complete.
Southern Bell, a luncheon for twenty-eight people; someone had written next to the reservation, in giant letters, and underlined, "NO SANGRIA".
As Anthony Grayling, an English moral philosopher noted in his book "Among the Dead Cities: The History and Moral Legacy of the World War II Bombing of Civilians in Germany and Japan," just five weeks after the destruction of Dresden, the British prime minister, Winston Churchill, expressed reservations in a letter to the British Air Marshal Charles Portal, chief of the air staff.
Some doctors who were concerned about the increasing public profile of CAM and the findings of the Smallwood Report expressed their reservations in a letter to the Times newspaper, outlining the need for effective distribution of resources on treatments of proven benefit.
Mr. Bronstein said he had no reservations about printing the letter or regrets about starting Mr. Penn's career in letters.
I arranged hotels and dinner reservations and took letters for luminaries like these and briefly they took refuge on my floor.
Initially, he expressed these growing reservations in private letters, mostly to younger humanists who were his own admirers but who found Luther attractive and assumed that because there were so many similarities between Luther's reform ideas and those of Erasmus, Luther represented nothing more than a powerful new voice in one and the same program of religious reform.
The company then said the agency's concerns were minor, until a cancer newsletter leaked a secret F.D.A. letter showing profound reservations.
