The Ship Costumes section of C-Store has been empty since Season 6 started, although it used to have many ship costumes available.
I bought them all long before Season 6 hit the server and since Season 6 I have been unable to use the Ship Costumes that I bought.
I sent a GM ticket about it and was asked to bug report this "previously unknown bug" as it was stated in the reply.
A bug report has been sent, but I'm creating this thread to hopefully get some more response from other players that has the same issue and also hope for some more response from Cryptic about this since those Ship Costumes are worth over 1000 C-Store that we have payed you to get.
I believe they removed them and put them in the fleet store to go with fleet ships however I though that if you'd previously bought them via the C/Z-Store they'd still be available via the ship tailor?