This trip flip flopped a bit so I wasn't able to confirm plans with any of the companies cargueiro kindly mentioned. I was also travelling light, so decided to try the mafia or the KFC trick as a fallback.

To my surprise the mafia were highly negotiable despite my total lack of Portuguese and the destination and I managed USD 30 for the ride. It took about 20mins in the evening rush so I didn't feel too ripped off. Still offensively expensive compared to Heech tho... which is pricing at USD2 for the return.