I mean, free in the sense that while I pay no additional money to what I'd already do, I exchange a mediocre lunch meal for a shitty one of diet mountain dew and a pack of Doritos. I've reached 10650 points. Still going pretty strong.
Just a few tips, although one of them may unfortunately be too late:
Get the 75 point Dorito Packs, they are much more likely to have 2 codes on the inside than the 100 or 200 points ones from experience. I'd say maybe 1 in 8? Conversely I havent' had 2 codes from a 100 point pack after having banked probably around 20-30 of them.
I signed up to the dewsletter and recieved an extra 100 points last night. Though it may have been that you had to sign up prior to last night...
Strangley if you're looking for $/point efficiency, the cheaper packs of either product is the better bet.
Commonly known but: remember to bank doritos and dew together for an extra 50 points.