I ordered a package December 25, and I'm using USPS Priority Medium Flat Rate Box, that's how it is shipping. And it said it would be here December 27-31. So what is the most likely date it would come between the 27-31?

Everying was closed on the 25th. If it was posted on the 26th then Monday or Tuesday are the probably delivery dates (though it depends a bit on how far it has to travel).

they would not have even seen you order until at least the 26th if they were even open then and it's possible it might not get shipped until Monday and if so, might not get to you by the 31st

It may come in between the two possible dates. Most likely being 28th or 29th.

you got to remember when using ebay you are dealing

with another person -NOT EBAY the company

so it depends when the seller ship it [now its possible

they are on hoilday and may not have shipped it yet