depeding on how green it is, put it in a squirt or sprite bottle. carefully take the cap off so that the seal doesn't break and all stays in tact.
like the bro's said, customs is pretty much a joke down there. they know you're comming to party and spend money. so they aren't too strict. they don't want to scare americans and their money away.
now getting back into the US is a different story.
i went down to mexico last winter and took about 20-30 pills with me. they don't care and they don't look too hard.
just use your head and you'll be fine!