It's in Blesta 4.0, which is currently in beta. When you upgrade to v4, go to Settings > Company > Plugins, and click to install "Mass Mailer". It will create a new Mass Mailer link under Tools.
That should only happen if there is no invoice or transaction has not been applied to any invoices,so if invoice exist and transaction is linked to it but amount is not applied than you may want to check settings-->automation-->Apply Payments to Open Invoices..and make sure cron is running