Page "Id Software" Paragraph 5

The shareware distribution method was initially employed by Id Software through Apogee Software to sell their products, such as the Commander Keen, Wolfenstein and Doom games.

They would release the first part of their trilogy as shareware, then sell the other two installments by mail order.

Only later ( about the time of the release of Doom II ) did Id Software release their games via more traditional shrink-wrapped boxes in stores ( through other game publishers ).