I hate Petco so much now. -_-" It's like I hear nothing but bad things about them, and I've only had bad experiences with them. My girl from them was sexed right, but they refused to treat her until I brought her home overnight and "proved" she was sick even though the lady working there KNEW she was! And then they tried to tell me I had to leave her at their vet for two weeks instead of them reimbursing me for a visit to a vet I know and trust... Ugh.
Anyway. I'd send them a complaint or something... idk. They're really just awful in general. lol.
I've always read that when a female is about to give birth to put her in a tank or bin cage with a mesh lid because babies can get caught in wire and such, or escape wire cages. Provide her with nesting material (like kleenex or papertowel) and don't have anything the babies could get on and fall off of to hurt themselves. Also feed her some extra protein to help make her milk more nutritious for the little ones.
That's all I can really remember... but I'm sure someone else with more experience will chime in! Good luck! It's still rather unfortunate, but at least you get the cuteness of baby rats in your home! lol