US Postal Service Rca interview questions

I applied 02/08/2017 and went to the interview on 02/25/2017 it was similar to what everyone else said, group then one on one interview with simple questions then was told to keep an eye out for an offer letter if we received one, on today 03/02/2017 I was given an offer of employment and a separate email was for an additional background check and drug test which I did the same day I got the offer..now beyond this point I don't know what else is to expect without any dates or any further emails like orientation, training and an actual start date

Casual and relaxed. It was a 1:1 interview process. The hiring person was friendly and welcoming so I didn't feel nervous. He answered questions and mostly asked yes/no questions. It was no more than 15 minutes long.