Do they really make you work 12 hours 6 days

Yes they do if you're a PSE mail clerk in the warehouse. Doing it as I type now.

So it's not mandatory 12hours for 6days?

It depends on whether you already have a permanent job before you begin with them. They Will work with your schedule but usually a 8 hr shift then if needed depending on the business of the day it maybe mandatory up to 4 hrs, but you cN let them know how long you can stay for