Postage prices are increasing. Priority mail envelopes are increasing to $6.65 and priority mail express is increasing to $23.75.
Be careful if you mail using stamps because the post service may return to sender or ask the buyers to pay the missing postage.
https://about.usps.com/news/national-releases/2016/pr16_086.htm
I feel like I've seen this post every year since sr1.