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Envelopes weren't used until the mid-1800's. Until then, folks just folded their letters, and wrote the name of the person to pick it up and the name of the post office. 

Stamps weren't used until 1847. Until then, if the sender paid, the envelope was marked "paid". If the sender didn't pay, the recipient was asked to pay when he/she picked up their mail at the post office. This changed in 1855, when the government got smart and wanted all mail stamped by the sender. The government had gotten smart and wanted their money up front! 

The post office first experimented with the railroad carrying mail in 1831 and let the first rail contract in 1838. The last railroad to carry first class mail was in 1977. 

Rural Free Delivery (RFD), the delivery of mail to the farm (instead of having to go to town to pick it up) was started as an experiment, criticized by some, in 1896. And wasn't established permanently until 1902. This act by the Post Office was probably more responsible than any other single influence on the building of good rural roads and bridges in the early 1900's, as the automobile made the delivery of mail to the farm possible. 

In the meantime, free delivery of mail in cities had started in the larger cities in 1863 and had spread to only 454 cities by 1890. The rest of the country had to pick up their mail, or the sender pay extra for delivery. 

The first time an automobile was used on a mail route was 1901. 

The first overland mail to California was in 1858, the Pony Express started in 1860, but was over with by 1862, when the transcontinental telegraph was completed. 

Parcels were first allowed to be mailed in 1913 (Parcel Post) leading to an explosion of goods available to rural America and the growth of Montgomery Wards and Sears as sellers by mail into retail giants. 

The History of the Post Office