I have sold a few DVD's recently - I posted them in padded mailer bags - 151 x 229mm (tight squeeze).

As you have about 300 to sell, if you bought the padded bags in bulk from an online packaging place that would be the lowest cost option.

Postage for one DVD in case, single disc is $1.40 letter rate, double disc in case - $2.10. This is for letter rate, no tracking. Those postage rates are Australia wide.

Must be less than 20mm in height for letter rate. If sending some that sell for a high, then you could send them registered post (with tracking) at extra cost on top of postage cost - about $3.50. 

If you did need to send a few to same buyer in  a parcel - up to 500gm satchel using eBay postage labels is $7.20 (plus ebays 9.9% FVF fee on top of all postage costs as well as item cost).