Postal History - Searching

When searching the who, where and when of postal history what websites do each of you use and pay for? Ancestry.com? Fold3?
Most postal history I collect and research is civil war but also may be World War I and II.
Thanks for any suggestions.

Yours seems to be potentially a two-part question, which perhaps you may want to clarify for us. An example you are stumoed on would be nice also..
There is the "postal history" facet of the postmarks, rates, routes, applicable regulations, and....
the "social history" facet, if you will, of the background of the sender and recipient, any contents, etc., which your post seems to be more about.

As a cursory reply, for background on the people involved, I almost always start with Find A Grave, various National Archives military databases, and just a normal Google search to try to get an initial sketch for further digging. As you would know for the civil war, many of the states have fairly detailed roster lists and regimental histories which go beyond Dyer's Compendium. Fortunately yes, if I want more detail the genealogy folks have the next level of information, which I can access through my local library without a personal subscription.

I always start with www.familysearch.org There is census data, marriage data, death data, and other data available. Its free site.