30 minutes to hold an offer is a bit rushed. I thought the norm was 24 hours. This is the part of post doc that I did not like. I actually preferred the more standardized match of internship.
Also, not NP here, so rules may differ, but two years ago you could absolutely tell a site they were your top choice. I am a detailed rule follower, so I know this was the case. I let my top site know by saying some almost identical to, "You are my top site." Their response was, "You are ranked very high on our list." Rules may have changed in the past two years, so take my experience with caution.