Maybe I can help a little. I had a feeling you were talking about Europe. It is a bit complicated considering the currency exchange.
I've read a book by a popular tour person in Italy and he said he takes credit card numbers, but does not place any charge on them. If the people cancel on him, he may make a nominal charge.
If I were you, I'd make a deposit via credit card for 2 reasons:
1. You have recourse should the tour director charge you too much or abuse the credit card information in any way
2. You've got some kind of guarantee that they'll show up to meet you at the arranged time.
Make sure you choose the credit card that:
1. Has the least expensive exchange rate for Euros to Dollars. Most bank cards will charge 3% whereas Target Visa only charges 1%.
2. Covers you the most.
If you've used a specific person or company, post that info. and ask for recommendations. There are some that are linked to this site that I've referred people to and they have been ecstatic with the private tours in Europe.