Well that's unfortunate. Even spaces, periods, and other common symbols? They are allowed on my forum through normal registration, so members were asking why they can't use it with the shop item.
The reason for this is that the input fields are similar to the ones used for colour and such, so extended validation needs to be applied to prevent XSS exploits in that feature.
I can look into separating the input fields somehow, but it's a significant undertaking so I can't guarantee when I have the chance to complete it unfortunately