Mike, if you have concerns about it being mis-sexed and they guaranteed it as female (hopefully you got that on paper or on the receipt) you should likely inform them immediately. Most retail places have a limited return time and reason so you want to inform them ASAP. Also if it requires a vet cost to confirm gender, I should think they should have to cover that or replace the snake with a female that they sex right in front of you. I'd be on the phone with the store manager as soon as they open their doors tomorrow.