Additional Monthly Transfer charge?

Regarding Additional Monthly Transfer $3.00/GB per Month.

If I a month go over the 5 GB limit by say 200 MB am I going to be charged $3.00 or only for the amount I have gone over the limit?

It will be $ 3.00.

instead, you can upgrade to the next plan

Can one downgrade again if wished without any additional charges?

yes, you can downgrade without any additional charges