I'm having trouble with this sort of thing at the moment too, I tried to download the new viper and it would let me actually download it but when it came to installing or loading it after the download it told me to try again later, tried it a few times but no luck, spose it was fairly lucky that i hadn't actually bought anything
Does it say it after the download? If so, it should be fine, mine does it each time I download something, but it is installed anyway.So does mine. Makes no difference to the content downloaded, it is just an error message bouncing back from the server due to some kind of small technical fault. You should still be able to download and purchase the car packs etc in-game.
Make sure when you download a car pack/individual cars etc, that you keep the download window open until complete, as there have been problems with the download not being recognised unless the active download has remained open until complete.