Where is the "Additional Parking" at the Cross Border Xpress (CBX, on the San Diego side)?

Question

I read on https://www.crossborderxpress.com/en/parking:

Additional Parking - $15 USD

Where is the "Additional Parking" in CBX?

At first I thought the "Additional Parking" may be the unlabeled parking lots on the following diagram, from the same webpage:

However, looking at Google Maps, some of these unlabeled lots are mostly empty:

And there are many parking lots nearby as well.