If you had 20 agents each in 20 cities, dropping off, say, 20 ballots per day each in 20 different mailboxes for 20 days, thats 20^5 or 3.2 million votes with 4

If you had 20 agents each in 20 cities, dropping off, say, 20 ballots per day each in 20 different mailboxes for 20 days, thats 20^5 or 3.2 million votes with 400 agents. That certainly seems plausable for a state actor or large NGO and seems difficult to detect. The difficulty would be either "harvesting" the unfilled ballots undetected or reproducing ones with the correct digital signature (I assume the barcodes on the mail-in ballots are unique, securely generated, and authenticate the recipient), which again would not be implausable for a state actor or large NGO with agents in the right place, the biggest saving grace being that different states have different systems. However, with national popular vote, you just need to crack 1 state system to skew the popular vote.