This somewhat absurd situation happens as follows.
I have paid the full balance due up to the this month.
My balance due on the statement is $150.
I return an item worth $50 after receiving the statement.
I go to pay my previous balance but I am limited by the current balance which is now $100. I pay the current balance of $100.
I now charge $200 before the closing date so my current balance is now $200. My new balance will be $200 on closing date.
Will I be charged interest during this period since I did not pay my previous balance of $150 (even though I wanted to pay it)?
Note that if I sent them a check they could not block me from paying the previous balance. They would have to accept the check. This only happens with online payments.