All the ships are available to anyone for in-game credits. You cannot buy ships with real money. That would be pay-to-win and let's not go there please.
As the others have said, you bought a skin for a ship you may have in-game.
Say you have a Hauler in-game, you can then go to the FD store and buy a nice paint job for it that'll cost you a couple of quid.
Then when you're back in-game you go to the outfitting area and select the Livery tab. In there there's a box for paint job. Click that and you can apply the skin you purchased to your ship.
It works the same way for all ships.
So if you have purchased the game but do not yet have a Viper then you're not going to be able to use that skin you bought. If that's an issue then have a word with FD and see if they can do you a refund. If that's OK then you can simply buy a Viper in-game as long as you have the credits and apply the paint job to it.
If you've not yet got a Viper in-game then best you get to work earning those in-game credits through trading, bounty hunting, or being a criminal. Then buy a Viper and apply the skin.