[mothered]

I hired an "insider" who instantly obtained a refund
I received a notice from Amazon stating that they plan to charge me again for the same order

It's not surprising.

The majority of refunders are only concerned about their cut, and couldn't care less about the ramifications on their clients. Stick to traditional methods - all have a proven track record of success.

Amazon managed to charge me even though my card was inactive.
Can amazon legally do this?

Yes.

Given you owe money to them, they have every right to collect It.