Well, I still say you guys should drive your own cars. It's not that bad, and you'll have a whole bunch more flexibility. Renting a car in Mexico sounds way sketchy, although I've never had to do that. I can just imagine signing something that says you owe them $1000's of dollars for a popped tire. No thanks. And I've rented cars in Costa Rica, so I'm not necessarily against the concept of foreign rentals per se.
That being said, I've never taken public transportation across the border either. I image that you might have to walk your stuff from one place to the other, then get your travel documents (Tourist card) at the Immigration Center on the other side. Count on an hour at Immigration. You'll have to ask a taxi company if they will get you across the border.
If Jonny and Smokem are already down there with a vehicle, take the Directo bus from Renyosa to Valles- you won't have to drive & get lost, and I'll bet it'll end up being cheaper than renting- gas is about $4 per gallon there. The Directo buses are like the gambling coaches here- plush seats, plus you can watch Weekend At Bernie's II en espanol the whole 7 hours down to the Valles bus station.
Regardless of what you choose, make sure you read all the immigration stuff before you leave: http://www.sanbornsinsurance.com/entry_customs.html
You need to have a passport or a notarized copy of your birth certificate to go into the Interior (60km into the country).