It is highly unlikely that “the greatest country on Earth” is unable to organise voting by mail. It has also been debunked several times and there’s no proof whatsoever this happens.
Why would anyone go through all the hoops and take the risk of casting one (!) extra vote?
It’s also very intriguing it’s only one party claiming this fraud is real, as if everybody that would commit this fraud, surely votes for the opposing party.
In my country in Europe, voting is mandatory. Booths are open on Sunday, and are located in your own neighbourhood. Waiting times vary between 0 and 15 minutes. It’s all not that complicated to organise.