Do you have to accept a postdoctoral position if the university has paid for accommodation and international flights for you to come and present?
If you are getting excellent treatment, that likely means they are fairly keen to hire you.  They are working to recruit you.  Accepting nice accommodations and amenities from them during the hiring process places no obligation on you--none at all.  That being said, this sounds like it might be a really good place for you.  Use your judgement about what will be optimal for yourself.