How much does it cost to mail a 1.5 oz envelope?

It is best to take it to a Post Office. Postage can depend on the weight but also on the size and thickness of the envelope, the destination, what is inside, etc.

As an example, though, a rectangular envelope (standard letter) with paper in it only, with stamps, that weighs 3 ounces, sent from Long Island NY to Saint George UT on January 29, 2016 is quoted on the USPS site at 93 cents. You can try out other scenariOS at postcalc.usps.com.