Paying the money would be a reasonable remedy but you need to have them provide an adequate release of all claims prior to whatever date they contacted you simply to prevent any subsequent actions.
I caution you to not be disrespectful or adversarial in your request simply due to the fact they have no obligation to first, not sue you and second, provide such a release. As business people, they will understand the request for a release of claims for actions prior to their contact for the payment you are willing to send them and should have no problem providing such a release.
If you have any stock left, you might offer to allow them to pay the shipping charges to have them delivered to them, if they would care for that. Obviously they are not going to reimburse you for them though.
Now, if you want, you would have an action against the party that sold them to you. If you are considering such an action, I would request to reserve one of the units as evidence for the subsequent court action that may ensue. Other than that, I would want them to understand you have no intent on selling an illegal copy of their product again and the offer to send the remaining stock to them would be a show of such intent.
btw: you are the third person that has been involved in a scam involving p90x that I have seen on this forum. I do not know where this scam is coming from but if it were me, I would seek to report this to any government agency that might be interested as well as offering to assist the rightful owner in an endevour to find and prosecute the scam originators.
