From the Mint's Twitter account moments ago
They added an option to ship between 1 and 3 oz items! Sounds dumb but this is actually awesome.
Saves up to 70% vs. USPS® First Class Package Service
Available for trading cards, coins and currency, postcards and stamps
Integrated tracking and easy to print using eBay labels
Shipping protection plan up to $50 on combined items, in case of loss or damage
Drop off at regular USPS locations
No smaller than 3.5” x 5”
No larger than 6.125” x 11.5”
No thicker than .25”
Weighs up to 3 oz.