"Snail Mail is 30 Times More Efficient than Email"
...and at least 30 times more expensive.
Mailchimp campaign:
1000 mails sent
40 people open it
10 people visit the website
cost: $15
Snail mail campaign
1000 mails sent
100 people open it/read it
50 people visit the website
cost: $1500
It may be more efficient from a conversion perspective, but it's certainly not capital-efficient.