I'm very much against people buying and using "shock collars" without devoting conscientious, significant efforts to understand the different types of learning and the huge differences between using rewards vs. using punishments. (While later I was a clinical psychologist, my PhD training was mostly in learning theory and my dissertation and research also were in learning theory.)
I've bought and use a Dogtra Pet IQ collar on Zach. It has 3 different types of stimulation and it has an adjustable intensity level that controls all 3 stimulations -- brief shock, "nick" shock, and vibration.
I put it on Zach whenever we go out for both of our 1 1/4 mile morning and evening walks when I throw a bumper for him to retrieve and get a few kibbles reward. Zach is very easily distracted by smells (especially anything edible) and over the hundreds of times we've gone for walks, I think I've only used the "Nick" on him once or twice and never its longer "brief shock". Every few days (less now than before, maybe once a week?) I do give him a brief vibration when he gets too distracted and isn't responding to his task of retreiving. His DNA is only 1/4 Lab so I'm hoping our morning and evening walks with retrieves eventually will build up his interest and delight in retrieving since it's such good exercise for him.