Just so you know, I am a British ex pat, moved here a year ago and so have no credit history in the USA.
I understand that they wanted a deposit, my point is that they should have made that clear before adding it to the bill...
I was going to point out that was probably the scenario given your user name and language....
I agree it should have been, "we will need to obtain a $400 deposit, if that is agreeable then call or do something to accept and blah blah blah" rather than just sending you a new bill with an additional $400 on it.
My electric company recently decided the way to ensure i pay on time is to charge me a $768 deposit, despite the dude telling me on the phone i have an excellent, consistent payment history. Thank you ex-husband for refusing to pay that last bill you were ordered by the court to pay.... The gift that keeps on giving.
Anyway, I'd be ticked too!