You don't actually have to buy something with credits. Because, why buy if you can get something for free.
So do that:
1. Go to the main menu
2. Select Forza Motorsport 3
3. Select Storefront
4. Select buy Tune/Design/Vinyl Group
5. Press X until the list is sorted on Cheapest First.
6. Buy one of 0 credits, or more expensive. But I'm cheap, so I paid 0 credits.
7. Get 10g.