Unexpected Plan Change: Will I Be Charged and What’s the Cost to Upgrade?

i just added 500 fast requests (to 1500) and then went to downgrade back down to 500 fast requests from the website so that I do not get auto-charged next month, but it automatically changed to 1000/500 fast requests… Will I be getting charged for this? Intuitive behavior would be that it would have kept 1500 fast requests messages until the following month.

Also, if I wanted to add more fast requests - since I am at 1000/500 - would I have to pay 40$ just to be able to? (20$ would only make it 1000/1000)