MySQL Database, establish with more fields than you think you'll need ?

I'm creating a site with user registration and a store for electronic downloads. I'm going to use WebAssists e_commerce database, as my starting point.

I also want to offer, in my store, licenses to use my online products for a period of time.

As an example, I could have a product that my users login to, which is an online flash application. The user will have purchased a license to use this product for a set period, from my store.

Do you have any suggestions as to the fields I should add to your e_commerce database to allow me to do this ?

What about a DetailLicenseExpiry value in the OrderDetails table ? or should I put it in the Products table, or should I put it in both on the basis that if I don't use that field, its no big deal, but better to have the fields in place before I start creating my PHP pages.

Any words of wisdon will be greatly appreciated.