I question whether the vote coercion/purchase opportunities opened up by mail voting are actually "huge" compared with in-person voting. But even if so, these opportunities don't scale easily and potential damage is limited.
If some malefactor wanted to buy 1 million votes at an average price of $50 each, that's $50 million of somewhat traceable money and 1 million people, some fraction of whom are certain to reveal they were paid. It won't remain a secret, and it will leave numerous evidence trails.
Contrast with fully electronic voting fraud - scalable, hard to detect, and (when well-executed) leaves little evidence to support nullifying an election and forcing a do-over.
If some malefactor wanted to buy 1 million votes at an average price of $50 each, that's $50 million of somewhat traceable money and 1 million people, some fraction of whom are certain to reveal they were paid. It won't remain a secret, and it will leave numerous evidence trails.
Contrast with fully electronic voting fraud - scalable, hard to detect, and (when well-executed) leaves little evidence to support nullifying an election and forcing a do-over.