Food for thought: if requiring a stamp to mail in a ballot is a poll tax and thus against the 24th Amendment, then what is requiring people to take time off work in order to vote?
In-person vote fraud happens, but rarely. So does mail-in vote fraud, but also rarely. The question isn't whether mail-in voting has fraud, but whether the fraud is any worse than in-person voting. And the answer is a resounding "no."
If you're liberal and unwilling to vote unless you get a unicorn candidate, you're part of why the country has slid to the right in the last 40 years. Vote every time, especially when you've got a choice between a great horse and and a slime mold.