I believe an $18,000 bill on a payment plan with $136 of monthly fees is similar (right now) to paying back an $18,000 loan at 9% interest (12*136/18000). I was thinking about the possibility of refinancing the bill with debt, but I'm not sure you'd find an unsecured loan for less than 9%. Paying it back with the current plan may be the best option for now.

edit: if the $136 monthly is a flat fee amount, then at $5000, $136 is the same as 32% interest. not sure if refinancing the debt is possible but at some point it may make sense, once it is paid down to the point that the effective rate is higher than credit card rates.

edit2: also, paying the minimum on that bill until it is paid off will take around 12 years (and $18,632 in fees), so getting to a point where you can pay more than the minimum will help a lot since you'll be avoiding that extra $136 per month.