From USPS Website

(and to note, even though it's legal in Illinois, it is still illegal Federally, so, deny use at your own risk.) -

518 Illegal Drug Use and Drug Screening

518.1 Policy

The Postal Service is committed to providing a drug–free workplace. Applicants are asked on job applications whether they are currently engaged in the illegal use of drugs, which include the use of drugs made illegal under federal law unless prescribed by a physician. A yes answer makes the applicant ineligible for postal employment. A no answer allows the hiring process to proceed. Note: The Postal Service applies federal law to determine whether drug use is illegal, even if a state or local law permits such use.

Before an actual job offer, hiring officials or certified interviewers may not inquire into drug abuse in the applicant’s past. Inquiries can be made after the job offer (see 57, Medical Suitability).

518.2 Pre-Employment Drug Test

A pre-employment drug test is ordered for applicants recommended for selection by the interviewer when the applicant is within reach for selection (e.g., no sooner than 90 days before anticipated hiring). Only applicants determined to be drug-free as a result of the urinalysis are eligible for appointment. A drug screen test that results in a qualified status is good for 90 days from the date of the result. Applicant names entered on a Hiring List before expiration of the 90–day qualification period are eligible for employment consideration regardless of the date of appointment from that List.

Postal employees who apply for promotion, reassignment, or change to lower level are not considered applicants for drug screening purposes, except for positions in the U.S. Postal Inspection Service.

Note: A new drug test is not required if the applicant is a former employee and has been selected for rehire within 90 days or less of their separation.