they can e-mail them, they can USPS mail them, UPS, Fed EX, send a telegram, walk down to your house, yell out the window, or have your mom tell you.
All they are doing is putting you on notice and making a demand. There are no requirements for how they deliver such a notice unless it is required for some legal action that may follow. If there are no such requirements for anything they may consider, it doesn't matter how they deliver.
Bottom line; you were given the opportunity to avoid a costly situation simply by returning yourself to the same situation you were in before you sold the DVD's, without the profit.
It's not like you are losing anything on this other than your eBay costs.
You have a choice of doing that or quite possibly defending yourself in court, hiring an attorney at thousands of dollars and still owing them thousands of dollars on top of that.
which sounds better to you?