it also then says:

letter mail

means articles that are small letters, large letters, articles lodged under the
PreSort Letter service, Clean Mail Letter Service,
Impact Mail, Acquisition Mail,postage prepaid envelopes, international postage prepaid envelopes, local delivery service or print
post service.

If you then look up what article means it says any object or thing

http://auspost.com.au/media/documents/AP-TCs.pdf