Abstract: A method performable over the Internet is disclosed, wherein a prospective purchaser is presented with a series of product configuration choices. A computer performing the method receives a series of product configuration choices from the prospective user, consults a database to determine whether the selected choices result in a permissible combination, and displays advice to the prospective purchaser about whether the choices result in a permissible combination and, if not, about the nature of the impermissibility. The computer then receives a new series of product configuration choices from the prospective user, with the new choices having been affected by the prospective user's consideration of the advice about the nature of the impermissibility of the previous, impermissible combinations of product configuration choices.