What you can and can't do with a commercial typeface is outlined in the End User License Agreement that comes with the typeface.
Typically, most fonts can be used as you see fit once you purchase the license. Some have restrictions, however, in regards to the extent that the typeface is the product.
So, for example, a font used on a t-shirt, likely not a big deal. Using the font to create custom stamps? That's probably going to require a separate license.
In the end, we can't answer this question in a general manner. It's going to be specific to each individual font. If the EULA isn't clear, just email the foundry. I'm sure they'd be glad to give you an answer.