Thanks for that. I didn't get an email, but the website lists similar promotion, once logged in:
https://buymiles.mileageplus.com/united/index_en.html

It's $20/1000 miles, if buying 30000+ (the promotion I see is they add an extra 75% more miles after 30000+); with fees it comes to $21.50/1000.

It's still higher than what they offered up until recently when booking flights, and going into "Manage Reservations", which used to run at $19/1000, but is now $21/1000 miles.