Send or print membership invoices
I see 2 ways:


Do Use Scheduled Reminders (Administer - Communication - Scheduled Reminders), If you choose Memberships as Entity you can use Membership tokens in the email being sent. You can limit the recipients per group. So if you want to send it one-time, just search for the people you want to send it to, place them in a group and create a reminder with just that group. Let the reminder be sent and remove the reminder.
Use PDF invoices (enable via Administer - CiviContribute -> CiviContribute Component Settings). You can then search Contributions and send individual pdf invoices per email to the results of your searches.