Can you reject a postdoc offer after the PI has paid a large sum for flights/accommodation for your visit?
It sounds like you are in the interview phase with no stated commitment in the form of having accepted an offer. Having your travel expenses reimbursed for an interview is not in any way a commitment to accept a position should it be offered. This is a completely normal expense that's part of finding good candidates. It is not expected that everyone they offer a job to will accept. 

As long as you approach the interview in good faith, meaning that you haven't already accepted a job elsewhere, and that there is some reasonable non-zero probability that you'd take the job if offered, you shouldn't feel bad about having your interview expenses covered because of the possibility you might not wind up there.