Patch 18.6.14 added GRACE (Grid Access Credit Extension), which is a $14.95 item that can be purchased and sold on the in-game market. When used, GRACE adds 30 days of subscription game time to a player's account. Unlike other items on the market, GRACE cannot be taken out and mailed separately. Also, since free players can't access the market, they'll be unable to purchase GRACE from other players.
Funcom has a FAQ covering the ins and outs of this new system.