Avis doesn't have this capability. You can theoretically change the vehicle they assign you in their mobile app, but most folks will tell you this functionality rarely ever works, and even when it does, the choices are typically meh. Hertz is much better in this regard--not nearly as good as National per see, but better than Avis and almost always much less expensive than National.
And yes, you really can skip all the lines and go directly to the PC section if you book with your Hertz Gold member number.
After booking, be sure to use the link below to track your rental with AutoSlash for a better deal. You can enter your Hertz Gold member number on the checkout page when booking through us to skip the counter just as you would if you booked direct.
Safe travels!