To allow a client to select a design template when a specific add on is added to a booking, just edit the design collection to include the add on.
Start by goin to Manage --> Design Collections and choose the design collection you want to edit:
Choose the Settings tab for the design collection
In the Applies to section, click Add ons and select the add ons that must be ordered for this design collection to apply to the booking
Click Save
Once your changes are saved, when a booking includes one of the add-ons you've selected, then that design collection will be available for the client to choose a design from.
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