From their website:

New Customers - Enter promo code FIRSTFREE at checkout for Free USPS First-Class Shipping on your first order through June 30.
Terms & Conditions

New customers must use a United States shipping & billing addresses in order to be eligible for free USPS First-Class shipping offer.
Valid once per customer and may not be combined with other discounts or promotions.
No cash value.
Expires June 30, 2015 at 11:59PM PDT.