I don't know about your post office but I'm pretty sure all of them are closed on Sundays. It being New years day they will be closed Monday as well. You won't be seeing your Betta until Tuesday if then.
Chard is correct. My husband works for the US Postal Service. All US Post Offices are closed on Sunday's and do not deliver any mail, even Express. Since New Years Day falls on Sunday, they are observing the holiday on Monday. If your seller were to send it tomorrow (Saturday) then it would arrive Tuesday at the earliest. However, based on the holiday weekend, the seller may decide to wait until Tuesday to ship him so he doesn't sit around in a post office for a few days. This would mean you would get him Wednesday at the earliest or possibly Thursday, depending on when the shipper physically delivers the fish to the postal service. For example, if you live on the West Coast and the seller is on the East Coast, then the seller needs to deliver the fish to the post office prior to 11:00a to guarantee a next day deliver clear across the country. But if the shipment isn't that far, then the seller can deliver it to the postal service later in the day and you could get it the next day. Unfortuantly "live animals" are not guarantee for next day delivery either. We usually receive them the next day, but the postal service has to handle them differently to insure a safe trip. So in reality, they can take two days to insure a live delivery. Worst case is the seller sends it Tuesday and you get him Thursday. Sorry for the long rant. Just filling in the details.