Since she was shipped without heat, that means she's had some chill, so both for that reason and because she was shipped (that's stressful with or without heat) as is
being re-homed, I'd wait several days at least, just to let her adjust. I'm not saying you need a full 5 days, if she's looking ravenous in 3 or 4 days, you're probably OK
to feed, but do give her a few days to settle in. Rat snakes have a lot of personality, they're usually quite hardy & she's adorable btw...so congrats on your new snake!
I'm a long-time fan & keeper of many rat snakes too...