The ABN#xxxxxxxxxxxx Code:PAID added to eBay addresses in Australia is reportedly for the purpose of marking parcels with GST tax paid. Initially, sellers were required to manually add the ABN number.
Until the change, the impact was minimal because it was possible to send stamp by First Class Mail International, a technical violation of UPU rules banning "merchandise" from First Class mail. If a stamp must be mailed in a First Class Package the cost jumps from $1.36 for a rigid 1 ounce envelope to about $14 for a minimum weight First Class Package.
Does this mean the end of selling stamps to collectors in other countries?
According to stamps.com:
Merchandise No Longer Allowed in First Class Mail International Flats
If you ship merchandise abroad using USPS First Class Mail International Flats, there's a new rule going into effect soon that you need to know about. Effective Sunday, January 21, 2018, First Class Mail International Large Envelope/Flat service for merchandise will NO LONGER be available from the USPS. First Class Mail International Flats will only be approved for use when sending documents. This change is occurring to comply with Universal Postal Union requirements.
Here are some examples of what USPS considers a document (still OK to ship using First Class Mail International Flats):
Audit and business records
Written instruments not intended to be resold
Money orders, checks, and similar items that cannot be negotiated or converted into cash without forgery.
Here are examples of items that will NO LONGER be allowed to be shipped as First Class Mail International Flats, effective Jan. 21, 2018:
CDs, DVDs, flash drives, video and cassette tapes, and other digital and electronic storage media (regardless of whether they are blank or contain electronic documents or other prerecorded media)
Collector or antique document items
Printed educational or test material
Player piano rolls
Commercial photographs, blueprints and engineering drawings
Film and negatives
These goods are dutiable and must be must be shipped using First Class Package International Service. Once this change goes into effect, shippers will need to include Customs Forms and the recipient could pay a duty or tax to receive the product.