At The Post Office, the nice clerks will let you know WHICH mail option is cheapest (MEDIA, 1stClass , Priority, etc)
First Class Mail is lower cost (cheaper) than MEDIA Rate UNTIL YOU GET TO A CERTAIN WEIGHT. For Example 6 or 8 cdrs just wrapped in a paper towel and wrapped in White Blank Copier paper with clear pkg tape is cheapest going First Class.
20 cdrs stacked and wrapped the same way will go MEDIA Rate at lower cost.
There is a minimum package size requirement that The Post Office has. Just as long as you SQUARE the dimensions of Round CD, with a bit of thick paper/stiff magazine cover piece/or some other paper stock from the wastebasket.
The Post Office people are required to ask on every package: Is there anything Liquid, Fragile, or Hazardous? They don't think you are a criminal. You don't need to use nice, new bubble packages. The Post office does not care if it is old reused recycled packaging or homemade package, so long as it complies.
One Thing, if you are taking your package to the Post Office and YOU WANT TO DETERMINE THE LOWEST COST METHOD, do not write anything like Media/1st Class/Priority on your package yet. LET THEM DETERMINE WHICH METHOD IS LOWEST COST and then they will rubber stamp or affix stickers AT NO ADDITIONAL CHARGE for whichever method ( MEDIA / 1st CLASS/ Priority )
The notion that MEDIA RATE is Very Slow is WRONG for all cases. For example if you are mailing to a neighboring large/medium metropolitan area within 300 miles, it should not be any different than 1st Class, at least that has been my experience. I've been mailing tapes/etc for 35 years or more , back when MEDIA RATE was Special 4th class Rate or whatever.
The Postal clerks in my area post offices are always the most helpful and knowledgeable and do an outstanding job. I wish more govt employees were like some of the postal employees in my area.