Just Received Official IRS Notice from Austin TX After Transcript Change - Anyone Know What This Department of Treasury Letter Means?

Just checked my transcript and got a weird code I've never seen before. The amount changed from what I filed, and there's this notice coming in the mail from the U.S. Department of the Treasury/Internal Revenue Service from Austin, TX 73301-0003. It's marked as "Official Business" with a "Penalty for Private Use $300" on the envelope. Really need this refund but no idea what's happening. Seems like the IRS is sending me something important that might explain why my refund amount is different than what I calculated. The envelope looks pretty official with all the Department of Treasury markings. I'm worried this might delay my refund even more, and I was counting on that money. Anyone gone through something similar this tax season? Has anyone received this same type of official notice from the Austin, TX IRS office and can tell me what it might be about? I'm getting anxious seeing the "Penalty for Private Use" warning on there too.