Back to the place that did the original work, and have em do it right.
From your description, you have pad(s) that aren't secure in the caliper, and or the pads were never bedded in properly.
I know when I changed ours recently, the pads came with a package of shims for the pads. There were instructions noting that the pads had to have the clearance checked between the pad itself and the caliper. There is a maximum of clearance allowed, .010-.015, above that and the pads will move around too much.
Also as stated, and our pads came with the instructions for doing this, was how to bed the pads to the rotors. Don't do it right and the pads form a glaze on the surface, and tend to be noisy.
Lastly, the quality of the pads can effect this, pads with no chamfer on the leading and trailing edges act like a plow as the rotor spins against them, causing them to squeal like fingernails on a chalk board.
The shops most likely are dumping a bunch of grease on the sliders, this dampens the vibration for a bit, but as soon as the grease wears down, back comes the noise.
Some techs out of laziness, grab the pads out of the box, and slam em in the caliper with no thought of what goes into a proper brake job. For them it's all about turning hours. Faster they get it done, more money they make. All fine and dandy till it turns around and bites em in the arse, and they get to fix it for free.