Think of it this way, the Earth exerts a force on a pebble which is directed towards the centre of the Earth and this force is equal to its weight $ mg$.
Also, the pebble exerts an equal force just opposite in direction on the Earth.
Similarly, $\ q_1$ exerts a force on $q_2$, so does $q_2$ on $q_1$. Only a single force acts between the two charged particles. The only difference is the direction of the force is opposite for both the particles.
When we say the electrostatic force, it is the magnitude $ F = kq_1q_2/r^{2}$ , and not the sum of $F_{12}$ and $F_{21}$.
Just think, if you combine the two vectors what you get is $0$ as the forces exactly cancel each other.
So, the Electrostatic force is equal to the magnitude of $F_{12}$ (and of course $F_{21}$) and acts in opposite directions on the two particles.
Best Answer
Let's look at some clues as to what it probably meant at the time. The word is ponderomotive rather than pondermotive and is constructed like electromotive, magnetomotive, from ponder-o-motive. The [etymology][1] of ponder is given as
Therefore as an initial guess, it could mean the line integral between two points of a force that acts upon substance to give it weight; perhaps the line integral of the Newtonian gravitational force?
Book Googling 'ponderomotive' turns up a quote from Energy and Empire: a biographical study of Lord Kelvin
Hermann von Hermholtz and the foundations of nineteenth-centurey science by David Cahan
Page 11 of Eddington's Principle in the Philosophy of Science
Page 165 of a 1922 Bulletin of the National Research Council By National Research Council (U.S.)
So Minkowski meant the electromagnetic force on mass - the Lorentz force.