Maybe you have noticed this and maybe you have not? The USPS or the United States Postal Service does not sell stamps at a huge discount. You may get a few bucks off at Costco. It is not going to be 25% or 50% off what they are sold for as shown here:
Lower left-hand corner of the first roll of stamps. Imagine that! The USPS selling 1st class mail stamps for 16.99 cents a stamp? Yeah, right. Facebook allows this advertising scheme of cheap stamps to exist. Spend $40 dollars and get 15% more off also.