Postal voters are actually recipe for disaster, there's a reason even my country don't adopt that. It's very easy to obtain ID for those that won't vote and mail them in. It's actually harder when you actually have to show up with your face and ID. And they use a simple ink on your pinky finger that last for 1 day, indicating you have voted.
It's actually very hard to rig an election (assuming 2 parties are involved and there's witness all over). Most of the election booth are run by locals, and the counting is witnessed by everyone involved, you really can't somehow tamper with it. The tally have to be certified by both witness. Most who comes are local neighbours, they'd know immidiately if there's a bunch of outsiders suddenly voting, and even then their name won't be on the list.
So unless they come up with retinal scan which means 1 person 1 vote I doubt they'd be moving too far from the traditional come and vote mechanism.