His *actual* concern with postal voting is that, on balance, many Democrats will use it who would not otherwise vote. Republicans have been working very hard over the last 20 years or so to make it harder to vote, especially for demographics that vote for Democrats. Mail in voting means more votes for Trump's opponent, so he attacks it, completing making up the story about widespread fraud. That's all he cares about. Period.
Likely he's also setting up for a gambit to refuse to accept the election results and steal another term. I think its almost a guarantee that he will try - no idea what happens when he does.