Shopify/active_shipping — pull request #69 by jeffutter (2013-02-03 07:31:48+00:00)
Title: Add FirstClassMailType to USPS package.

Without specifying FirstClassMailType while specifying First Class as the service you get this error:

ActiveMerchant::Shipping::ResponseError: Invalid First Class Mail Type

According to the API docs you need to specify a First Class Mail Type when using first class.