The Print Science Designer has a client/server architecture: In this case, the client is the shopping cart system you are using and the Designer server is the server. Essentially, the shopping cart system (client), will request the the Designer server launch a design online session. The design online sessions takes place on the server. When the design online session is complete, the user is directed back to the shopping cart system (client).
For the most popular shopping carts, Print Science offers free-of-charge plugins that manage the dialogs between the client and server. When using one of the plugins created by Print Science, there is no programming required.
For shopping carts where Print Science does not offer a plugin, you can modify your shopping cart or prepare a plugin for your shopping cart that will access the Print Science Designer via its API interface. In this case, you will require a developer to perform the required programming.