The official USPS website allows you to place orders for stamps and shipping supplies as well as pay for and print shipping labels that you can stick on a package and give to your mail carrier for delivery. The government website is the only place to bypass third parties and conduct business with the USPS directly.
When there's a USPS discount to be had, Tjoos.com will have the Postal Service promo codes you need to save some money. Click on a USPS promotion link and you'll find yourself at the USPS website. Redeem your USPS coupons by entering all of your USPS coupon codes into the Postal Service coupon code box.