It depends upon the size of the letter. It can be large envelop, square envelop, envelop containing a rigid object or old shaped envelop. Let me tell you the rate of a letter which will be delivered between 3 to 5 days. The post office rate is 25$. If you want to send it between 6-10 days. The post office price would be 18.50 $. For more information visit,
Postage Rates
Postage Rates