Hello, @TheNewGuy-0614,
Welcome to the Microsoft Q&A platform!
To archive mailboxes without paying for the full E3 or E5 licenses, you can use the following options:
1.Exchange Online Plan 1 with Exchange Online Archiving Add-On
Exchange Online Plan 1 allows you to move old messages to an In-Place Archive, but it doesn't include the Litigation Hold feature by default. To enable Litigation Hold, you need to add the Exchange Online Archiving license.
2.Microsoft 365 Business Premium
This plan includes archiving and in-place hold features without needing additional licenses.
3.Exchange Online Plan 2
This plan includes both archiving and in-place hold features, but it is more expensive than Plan 1 with the Archiving add-on.
For reference about these descriptions, please visit Exchange Online Archiving service description - Service Descriptions | Microsoft Learn.
These options should help you manage archiving for important users or those leaving the enterprise without incurring the higher costs of E3 or E5 licenses.
Should you need more help on this, you can feel free to post back.
If the answer is helpful, please click on “Accept answer” as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.
Thank you for your support and understanding.
Best Wishes,
Alex Zhang