I use their "Advanced" billing mode which is usage-based and ended up cheaper than the fixed-price "Simple" mode. This was my bill for last month (received ~300 emails):
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
No need for a filter rule apparently!
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) […]"