Umm..you can't make payment plans to pay your premium payment. They expect that amount on that date or they cancel you. Just an fyi--it's not a final bill from a hospital it is a premium that is already broken down on a monthly basis ie 600 is likely the MONTHLY premium for their COBRA.
That aside, why is it sooo imperative that a 19 year old in good health have insurance anyway? There are plenty of minute clinics that charge about 60 bucks for common ailments likely to befall a 19 year old and most prescriptions need to treat such are on the free or 4 buck list at the pharmacies. Would seem far cheaper to take one's chances and not insure her for a few months. Other than that, why is her 19 year old hiney living at home if she's not in college(which would have eliminated this discussion altogether) or working with benefits somewhere to cover--*gasp* herself or at least to provide income to cover that cobra payment if its so necessary? This isn't anybody's fault buy you and your prescious little girl. Get her butt to work and if she wants insurance so bad, she, as an adult, can help pay for it! Geez, this isn't that hard to figure out.
BTW, I went from the age of 18 to about 23 without health insurance coverage...I had one 'big' incident...my wisdom teeth, which I made a payment arrangement for the anesthesia(at the time my dental woudln't cover and I didn't have medical(couldn't afford it). Paid the 500 dollar bill off in a few months.