Associate to an Account Task generated from and order

I need a reminder for invoicing that allow me tro track the amount, the invoice forecasted date and the supposed payment date. They can be more than one for each order, and I would like to track also for potentials. In any case they should be created before the invoice

I could do it quite fine with task additional fields, but I cannot find a way to have a report that links task to accounts if I create them from and order or opportunity. Any suggestion? (also for a complete different approach)

Thanks in advance