Add-ons are a separate category of game parts. As Space Agency grows, new rockets, modules, station parts, and other items are added to the game.
Periodically, a few select special parts are made available to players only by purchase for use. They cannot be unlocked by completing missions, like most other parts.
NOTE: In general, if a Sandbox containing an Add-on part is sent to another game player, they will only be able to open the Sandbox if they too have previously purchased that particular Add-on.
These premium parts are shown on a separate menu screen, depicted at right (prices are shown in U.S. dollars $).
Additional features are also made available for purchase.