Blimey Jim that's a bit off, for the customer to do that without any warning to you. I would give her a call and very subtly try to work the conversation in to her admitting she knew about the fleas, then whack a load on the bill.
Although I agree with the theory that a colloidal product should kill small insects I await keenly for any solid evidence. Yes, if you bathed them in it maybe it would be very effective, but spraying a very small amount on a carpet in hope they'll walk in it? Remain to be convinced
Get yourself some Ficam-W, follow the instructions and it should sort them.
If that happened to me I'd be properly cheesed off, to put it politely. I'd wrap the rug back up and go dump it on her doorstep, to hell with the job.