The character spent a feat to get this ability, and chars without the feat can shove as an action. Would an acceptable reading not be: the shield master's shove can be regarded as a bonus action instead of a regular action if she also decides to attack that round?
Well, a given DM can rule however he wants.
But my issue with it is that the attack action might never be used at all in a round, but the bonus shove might be.
For example, the PC bonus action shoves his current foe away. He then moves towards a different foe, but gets Held with a Readied Hold Person, never reaching the second foe. He got to use the Shove as a bonus, but never actually got to do an attack action.
I would rule the same on Eldritch Knight War Magic. Cantrip goes off first which allows the bonus action of the weapon attack. Ditto here. Attack Action goes off first which allows the bonus action of the shove.