Bob,
Sorry for the delay in response.
The use of the marking "PAID" began appearing during the "Stampless" cover period; it appeared in manuscript or handstamp form with "PAID" commonly incorporated into the design of a town circular date-stamp to convey to the receiving post office that postal fees were "PAID".
The "PAID" marking in question is identified as "PM-PF 12" used in BOSON, MASSACHUSETTS as early as 1861, illustrated on page 323 (see below) of "UNITED STATES CANCELLATIONS 1845-1869" by Hubert C. Skinner and Amos Eno, © 1980 Hubert C. Skinner, published by the American Philatelic Society, Inc., State College, PA.

The marking is a fairly common marking. On cover, the straight-line "PAID" strike is normally found with found with a red or black "BOSTON, MASS." double-circle c.d.s. cancellation. (See example below -not my cover).

Hope that answers the questions on the markings.
Best,
Hal