If I tripped and fell on my porch due to a package left by a delivery service, should itbe responsible for my medical bills?

No, the delivery service is most likely not responsible for the injury.

Responsibility depends on fault. Fault generally depends on negligence, or unreasonable carelessness. Whether an action is unreasonably careless has to be judged by the context. Delivery services--whether UPS, Fed Ex, private couriers, furniture delivers, etc.--leave packages on the porch when no one is home to get it. It is unlikely that a delivery service would be found to be negligent for doing its job in the way that delivery services all across the country do.