I take it you chose tour #3 Walking Provence West?
When I first read your post, I was thinking day walking tour, so it through me a bit when you noted the deposit amount.
Now, I see their walking tours are a week or more (the 'moderate' tours) and include accommodations, meals and transfers. I can see why they might want everything up front.
Still, I wouldn't feel good about paying everything up front unless frequent T/A posters could confirm their own experiences in a positive way.
That might make me feel better. Do you know anyone who has used this tour service?
Why not check with the Avignon tourist office to see if they have ever heard of them?