Generally not hard to find, these are interesting. Many are marked as transiting or sent from a T and T post office. Before 1808 they are not marked. Pre 1808, with few exceptions all mail within Germany and the Holy Roman Empire was carried by the House of Thurn and Taxis. This example, new to my collection, was mailed from Frankfurt an der Ulm (Not the larger and more famous Frankfurt am Main) to August Franz Friedrich Graf und Herr zu Castell (1705-1767). A short biography and portrait are online. He is addressed by his full title. In the lower left is the notation "Franco Frankfurt," indicating postage payment. An amount paid is not noted. It was mailed October 14, 1763.

This is part of my specialized Thurn and Taxis collection.