Basically you select a seat, then you have to issue an EMD against it.
http://en.m.wikipedia.org/wiki/Electronic_Miscellaneous_Document
I'm not a Sabre person, but I'll try to post some documention tomorrow
Haven't tried it yet, but here's the format:
View available Economy Plus seats via standard 4G entry; e.g., 4G1*
· A banner is displayed providing Economy Plus seat fee information
· Hover-overs in Sabre’s Graphical Seat Map showing individual Economy Plus seat fees
· Select / Book by clicking on the seat on the Graphical Seat Maps or making the standard 4G1/ seat
request entry; e.g., 4G1/6C
· The AE field is automatically added to the PNR and can be viewed with *AE and *AES
· Once status code is updated to reflect HD1/PAYMENT REQUIRED. Be aware of the PURCHASE
BY date/time in the *AES
· Ensure air ticket has been issued
· Status of the AE seat item must be HD (confirmed, pending payment)
-Entry to drive payment of a Economy Plus seat
If using same credit card as used for air ticket, enter W¥EMD*AEn (where n is the Air Extra ítem number)
-If using a new credit card for payment of Air Extra W¥EMD*AEn (where n is the Air Extra ítem number to fulfill) ¥F*VI4444333322221111/1216