How to allow free listing submissions and charge only for listing upgrades
If you want your users to submit listings for free but charge them for additional features like making the listing Featured, adding additional custom fields, ..., it is still required that you create a base plan because upgrade plans are not available if a base plan is not applied to a listing.
In this case you must create only one base plan and select Free or Trial as Payment type.
Make the plan default, and if you set Show in submit form to No, users will not see the plan when submitting a listing, but this free plan will be processed automatically.
This will make it possible for listing owners to Order Upgrade and pay only for special features that you created as listing upgrades.
Existing Listings (using bulk order generator)
If you create a listing plan for a category that already has listings (created before listing plans were available), all media and custom fields will be hidden in those existing listings.
For existing listings you need to create a free base plan that will include the media and custom fields.
When you save the free listing plan, you will see a Create orders button next to the plan. When you click this button, it will generate orders and apply this free plan to all existing listings that don't have a plan: