I would definitely recommend collecting mashua (Tropaeolum tuberosum) and oca (Oxalis tuberosa) plants while you're there. Varieties are difficult to come by in the U.S. Both are great tuber crops that can do well in poor, acidic soils and you can grow them all up and down the west coast of the US (and into coastal B.C. as well), and probably other parts of the US as well. You can grow them in Western Europe, New Zealand, and southeast Australia as well. I've grown them here in western Washington state and they're both really cool plants. Oca is sensitive to frost, so it's not a
perennial unless you're in California or in USDA zone 9 or warmer, but mashua can take quite a few heavy frosts and will come back each year. Oca tastes like a potato with a bit of a 'sour' bite, which is from oxalic acid, which is the same thing that gives spinach and sorrel their sour flavor. Mashua is less palatable, but is crazily productive (I got five pounds off of one plant), very beautiful (it looks just like a nasturtium, it's in the same genus), and I think it'd make a good livestock feed. It does have the weird effect of decreasing testosterone levels though.
One way you could try to get the tubers past customs would be to ship them off in the mail from Peru - they're might be less of a risk of them being destroyed that way.
@ Erica: What colors are the oca tubers from the varieties you have? I'm always really interested in new varieties, and if you have some directly from Peru, it's very likely they're varieties that haven't been grown here in the US before. How did they grow where you're at and what's the climate like there?
-Adam