It is not clear from your question how you have been overcharged. If you …
The key issue from what you write is that you say you "always guessed …
1) If you did not get what you had purchased, you may terminate the …
Yes, if they owe you money for some reason--including pursuant to some agreement with …
1) You are only obligated, in the first place, to the degree you actually …
Are you a lawyer?