If one rodent in the pet shop has mites, they pretty much all do. Pet shop employees don't wash their hands between handling animals. And the type of mites that affect rats are too small to see without a microscope, like the mites that live on your pillow and bedding.... way too small to see without a microscope... Lastly some rats don't scratch so they don't look like they have mites, we adopted one rat the never scratched but infected the rat we already had... about a month after we adopted the new rat our old one was infested.
Pet shops don't want to be 'responsible' for infesting your rats with mites... so naturally it's the wood chips...
Lastly don't beat yourself up about quarantine, like I said some rats don't ever scratch so they will sometimes go through the best quarantine unnoticed.
The revolution will kill the mites on your rats, who will in turn suck up all of the mites in your house and kill them too. That's why it's so good, it makes your rats mite killing machines for up to 30 days. This is especially important if your rats free range the house.
Rat mites are incredibly common, thankfully there's a quick fix. (and it isn't cat shampoo)
Best luck