Cheapest way for me, for one disc is through stamps.com, I print the label at home and tape it on the bubble mailer. If I ship it first class, it's usually 2.46-2.59$. First class is anything under 13 ounces I think. Or click-n-ship is another option from home instead of stamps.com. And if you don't have access to ship it at home, just make sure to do first class for one disc at the post office, they just might charge you for a bubble mailer.
When shipping two discs, it really depends on how far you are shipping it. I will normally use parcel post, but if parcel is more than 5 bucks, I will just upgrade it to priority and throw it in a padded flat rate envelope. Those envelopes cost 5.70 to ship for me on stamps and it get to the customer way faster, normally in 2-3 days.
I hope that helps. I can't stand to see the post office rip people off all the time, if you know what you want, it will be cheaper.