You are correct - certificates do not drop from the most recent packs. You can only obtain certificates from the older packs starting with Shipment 2: Contraband and ending with Shipment 7: Explorers. If you are looking for certificates, you have to hunt for older packs.
TorFashion has an excellent guide on cartel reputation tracks and which specific packs within a shipment to look for if you want to find certificates.
http://tor-fashion.com/cartel-market-and-reputation-guide/
If you have certificates, they can be used to buy certain armor or decorations on fleet (if you have the required reputation for vendors in the Cartel Bazaar).
