I prefer prepaid envelopes and buy them in 100 lots to save 10%, works out at $2.20ea.

These are the best for series as they usually weigh over 125gm and the prepaid envelopes will allow 500gm as long as it goes through the slot.

As said above, I use the larger prepaid envelope to post 2 dvd's or 2 series side by side as that way they still fit through the slot