To indicate the proper package type for your shipment you must use one of the following (in this format) in the field marked "package_type":
card
letter
envelope
thick_envelope
parcel
flat_rate_envelope*
flat_rate_legal_envelope*
flat_rate_padded_envelope*
flat_rate_gift_card_envelope*
flat_rate_window_envelope*
flat_rate_cardboard_envelope*
small_flat_rate_envelope*
small_flat_rate_box*
medium_flat_rate_box_1*
medium_flat_rate_box_2*
large_flat_rate_box*
large_flat_rate_board_game_box*
regional_rate_box_a_1*
regional_rate_box_a_2*
regional_rate_box_b_1*
regional_rate_box_b_2*
*Some options may only be available for grandfathered accounts.