Student or Learner
Teacher, is this sentence OK?
Thank you for your letter attaching your cheque to reimburse us for the cost of obtaining the documents from the bank.
Are you really thanking them for the letter? Or are you thanking them for the check?
In any event, "letter attaching your cheque" doesn't make any sense.
Thank you for sending your check to reimburse...
Or, if you must, "Thank you for the check attached to your letter, which reimburses us for..."