The United States set up the Parcel Post system in 1913, and issued special stamps for this new parcel mail. On July 1, 1913, these stamps became legal to use on first class letters, as well as for any postal payment. I've noticed that, by 1914, first class usages seem to be fewer. After 1914, Parcel Post usages are quite uncommon.
I've enjoyed collecting these stamps individually, as well as on cover.
For more information on these stamps and the service, take a look at an excellent website with Parcel Post stamp images. Also view my Google photos album of Parcel Post stamp images. More information on postal history subjects is available at the Stamps and Postal History Pages.