When importing shipments via CSV, you may only use one of the following (in this format) in the "postage_type" field:

usps_express*
usps_express_mail_international*
usps_first
usps_first_class_mail_international
usps_first_class_package_international_service
usps_library_mail*
usps_media_mail*
usps_parcel_select*
usps_priority
usps_priority_mail_international
usps_other*
chit_chats_canada_tracked
chit_chats_international_not_tracked*
asendia_priority_tracked
ups_mi_expedited*

*Service availability may vary.