I wouldn't use 2 first class postage stamps when sending a letter; you'll be overpaying. The rate depends on weight. These are the current rates:https://www.usps.com/ship/first-class.htm
* 45 cents for 1oz or less
* 65 cents for 1-2oz.
* 85 cents for 2-3oz, etc.
I'd say your best bet is to take your sealed envelope to the post office, have them weigh it, and stamp it. While there, buy a bunch of stamps with that same denomination. Use those when mailing from then on, assuming you always use the same number of sheets and same type of envelope. (Also be aware when they change the rates, which they just did recently.)