Jambo Natalea - to follow up on the name of the tour operator and yes I agree it can be scary handing over $500-700 US to someone you know nothing about. So do some research, ask for names of people who have been with them recently - to confirm what they are telling you is correct. Which bank are they asking you to send the money too? Ask your bank/financial institution to enquire about them. You can also do some online checking of their bank - you should be able to determine how big they are and how long then have been in business. I think 50% is a bit over the top - usually 30% is what I have paid. Another question how are they asking you to pay the balance of the fee? If its cash then be more than a bit careful carrying large amounts of cash. ATMs / Cash points abound across Tanzania and you can withdraw up to 250-300,000 TSh (per day) to pay the rest of the amount. Yes I know that national park fees for non-citizens are paid in US dollars - but they are actually paid by the TO via a specific debit card, if the driver tries to tell you that the card is not working and ask for "extra" funds (that he assures you will be refunded back in Arusha) tell him NO and to contact the TO directly (he will have a mobile/cell phone and will be in regular contact with them). Good luck