I previously knew about First Class
"First Class" is a USPOD/USPS domestic
term for "letter mail," the international term from the UPU (Universal Postal Union) which tends to control international mail and mailing rules.
There is no corresponding international "small packet" class of mail term in the domestic
While UPU international Parcel (actually 'Parcels
') Post started about 1888 from the USA, there was no domestic
USA Parcel Post Service until 1913.