The $5 off special can be used multiple times through May 31st. You just need to use the code: "CURBSIDE5" when ordering through the Panera website or app.
As far as their curbside service improvement goes, they've extended their free Wi-Fi radius to cover the area around each location and, with the geofencing (if you opt-in), they will be automatically notified when you arrive so they can prepare your order and bring it out to you. If you don't opt in, you'll need to tap "I'm here" in the MyPanera app or in your order confirmation email when you arrive.
Photo via Panera.