That letter weighs no more than 13 ounces.
The maximum weight for one postage stamp of First-Class Mail is 13 ounces (over 13 ounces, First-Class Mail becomes Priority Mail).
In January 2016, 14,15, and 16 ounces were announced as acceptable but only for commercial First-Class Package Service, not retail service.
The weight of your mailpiece is an important factor in determining the postage price you pay. USPS Rates are determined based on a combination of weight, mail class, and shape.
For certain services, discounted pricing is available for customers who purchase from online PC Postage services such as Stamps.com. Here's a summary of the latest USPS prices.
For more info, just log on to www.usps.com or follow them on Facebook and on Twitter for news and updates.