U.S. Postal Service - possibly. Any other private carrier - nope, they can do that.
Only mail sent through the Post Office is Federally protected, and even then, while they cannot legally open it, they can confiscate it and have it returned unopened to the sender. If the company name and address are on the label, they (company officials with power of attorney) have the right to refuse delivery (which is what marking it as 'return to sender' does).
As for the overtime thing - that's another large can of worms. If they do any business across state lines, they are required to abide by the Federal rules. Assuming that is the case, if/when you leave the job, report 'em to Wage and Hour; either you are exempt and your salary is supposed to reflect that (many companies don't pay attention to that, though), or you're not exempt and you are supposed to be paid for overtime, at either one or one and one-half times your hourly rate.
no subject
Any other private carrier - nope, they can do that.
Only mail sent through the Post Office is Federally protected, and even then, while they cannot legally open it, they can confiscate it and have it returned unopened to the sender. If the company name and address are on the label, they (company officials with power of attorney) have the right to refuse delivery (which is what marking it as 'return to sender' does).
As for the overtime thing - that's another large can of worms. If they do any business across state lines, they are required to abide by the Federal rules. Assuming that is the case, if/when you leave the job, report 'em to Wage and Hour; either you are exempt and your salary is supposed to reflect that (many companies don't pay attention to that, though), or you're not exempt and you are supposed to be paid for overtime, at either one or one and one-half times your hourly rate.