It depends on the type of mask. Bubble envelopes don't offer much protection for paper mache or paper clay. I would worry about leather work too.
I find the non-flat rate Priority Mail boxes work best. You may pay slightly more when shipping all the way across the country than the flat rate fee, but make out better more than half the time.
East coast states have been as low as $6.05 to $7.30. The most i think it cost me was to Hawaii and that was 11 or 13 dollars.
The USPS "shoe box" size Priority Mail box works. I go one size larger with the delicate ones so I can pack a box within a box.
If its under a pound you can go First Class too with your own box which sometimes runs only a few dollars to about eight.
The above info is for domestic mail.
