A drug test is no big deal if you don't use drugs... all you do is pee in a cup and give the cup to a nurse... it's tested for the usual suspects and you're done.
Does it sound like a legitimate job? Perhaps... but you're going thru an agent... this is the problem with doing your job searching on the internet... you aren't always talking directly to the company... You want to work for the postal service... contact the postal service - they will probably still drug test you -- but they won't charge you a 20 dollar filing fee.
Jobs a the USPS are here:
http://www.usps.com/employment/welco...age=employment and you generally have to take a test...