their is possessive, whatever it refers to belongs to someone. In this case it means the tours that the travel agency has.
However, it could also mean the tours that belong Mr. and Mrs. Douglas, i.e. the tours that they will take. The correct meaning depends on whether the Douglas' have already bought the tour or not.
when a possessive pronoun could point to more than one person or thing as an owner, you need to look at the context, if you don't know the context, there is no way to tell, but without context, the first one is probably correct, because a travel agency always has tours, and people who are there have probably not bought a tour yet. Also, people who go together can say we are going on a tour, but since there are two people you could also say that they each have a tour, so the Douglas' have tours (especially in the third person, like this sentence).
It really is a perfectly ambiguous sentence. Good question!
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