Hi I've done quite a bit of research into trekforce and similar companies, hope I can help,
My friend went with trekforce and had an amazing time, however it has got more expensive since she went. which is one of the reasons why I have chosen to go with raleigh instead. From all the organisations I looked at raleigh and trekforce seem to both have a good record and offer a genuinely valuable volunteering experience with an adventure element. Although they are both roughly the same cost, raleigh is a charity so you can fundraise a bit easier using the gift aid scheme (so you basically dont pay tax on what you donate). Though raleigh don't run a trip to the amazon, so if you're set on the amazon then I would suggest you definatly go for Trekforce.
There's a few threads on here already comparing the two. Other organisations that do similar things are ventureco, quest overseas.