Are you backpacking in terms of wilderness trekking with tent, food and sleeping bag (the focus of this site) or only in terms of budget travel (for which there are better, travel-focussed sites)?

In any case, do remember that May in Colorado is November in Chile (in other words, chilly--sorry, couldn't resist the bad pun! ) Also, if you are trekking up high, the altitude makes a big difference--as you should know from Colorado.

I'd take that puffy--or, as balzaccom suggests, shop the local markets for lovely warm stuff. Also the appropriate accessories for hands, head, feet--think Leadville (or higher) in November!

Had you thought of starting in Chile or Patagonia and working northward, so you aren't visiting there in the middle of winter? Although if you're leaving this month, you've probably already made your travel arrangements. In that case, think Leadville in January!