Under the current pay chart, annual pay based on a 40-hour week comes out to $40,734 at the bottom for a rural carrier. That works out to $19.58 per hour. If you were to be converted to career status, that's what you would be earning. Once you reach Step 13, and I don't know how many years it takes - for city carrier it takes about 12 years to reach top pay - annual based on 40 hours is $54,162, or $26.04 per hour.
Depending on where you are, you may be required to use your own vehicle - some offices provide a USPS-owned vehicle - and you would be paid mileage. Your route has a pre-determined distance and your annual pay is based on how many miles your route covers per day. Once you reach the 30 mile per day mark, annual pay increase per mile is capped at $35 annually per mile. The annual increases are much higher per mile at the lower levels.