So they don't charge for emails if they are forwarded? Do you have a link where they describe underscore aliases?
Total: $0.35
Storage (GB/month): 0.00 ($0.00)
Emails sent to non-Purelymail accounts: 19 ($0.00)
Emails sent: 19 ($0.00)
Emails sent (GB): 0.00 ($0.00)
Yearly account fee (days): 30 ($0.33)
Shared-domain user fees: $0.02
https://purelymail.com/docs/features
"Subaddressing allows you to tag the email addresses you give out. […] This also works with example_tagged@purelymail.com (in fact, everything after the first symbol- anything not a letter- in an address is ignored for routing) […]"