Check the online USPS calculator.
http://postcalc.usps.com/
Startlingly enough, it seems the rate is likely $7.35.
A dvd is too thick to go as LetterMail (max 1/4 in) and it would probably be disqualified from going Large Envelope since it would cause the envelope to be too rigid and it would contain "items that cause more than 1/4 inch variation in thickness". That leaves you with First Class parcel - and for 3.5 oz, that is $7.53
yipe...