So, after speaking with a HubSpot sales representative, I understand why these formatting tools aren’t available to many of us. To be able to format both thank you responses and rich text auto-reply emails, you need to have a paid HubSpot marketing subscription. I can’t remember if the starter package qualifies or if it’s a higher tier than that.
But given my organization’s status as a fledgling nonprofit startup, we will be having to create workarounds for now. Bummer.