DodgyPutter: Is that true? I could easily be wrong but I thought there were only ever three apparel upgrades on offer and they all cost credits
Yep, Marchie is correct. The free apparel, the ones you get from sponsor gift boxes (or cases), can be upgraded once you have the required amount but, you need coins to do it.
As you upgrade that apparel, the special enhancements the apparel provides are increased.
For instance, the Swing Edge hat provides increased distance. I started with a lvl 1 hat and eventually upgraded it to lvl 9. Each time I upgraded, it cost more, and more coins. My driver, with no enhancements has a distance of 282. With the lvl 9 Swing Edge Hat, it is now 305.
I believe, at this time, you can go as high as lvl 10 for each item.