Pay to flag a node with drupal commerce Please help achieve the folowing:
I have a content type "proposal" with a field "prépayment_amount". I want the user who try to flag this node to pay the amount indicated in the field "prepayment_amount" and when the payment is successful, I want to notify this to the user who created this node, if not the flag is cancelled.
At least a partial answer could be to use Commerce in order to pay for access to a node. A user who clicks the link will be sent to the pay page, and upon completion of payment, the user will be assigned the role which can "flag" that proposal. 

I'm not sure this would end up using the flag module, but you could theme it to look flag-ish.