The voting period in Washington is 18 days. So someone would have to either be constantly looking through my mail for the better part of three weeks, or have the best luck when it comes to predicting when I would actually send it out, and that's if they got the correct mailbox and I didn't, say, go to a post office or the ballot dropoff boxes. It would be hard to do even for an individual, and much harder to scale up.
If you picked the right districts, it wouldn't even cost that much to tilt the votes in your favor. At $50/ballot you'd almost certainly get 11,000 extra votes in Michigan - enough to swing the state in the last election at a total cost of just over half a million dollars.
This ease of abuse is one of the many, many reasons that I support a single, national vote for president over per-state winner-take-all electoral college voting.
It’s difficult to scale without getting caught.