United States Postal Service 150 3rd St, Wentzville, MO63385 The Wentzville Branch of the USPS is conveniently located right in the middle of the old business district. Post office…More boxes are available for rental in various sizes from personal mail to a large box for larger businesses. It can be busy at times with a line out -the-door but employees keep it moving right along. Obviously they have an array of choices in stamps and collectibles. It also has packaging available on-site.