What's the best shipping option for an online store?

Hi there. I am in the early stages of starting my own online store. Shipping is obviously a big part of it. For someone who will be shipping out a lot of things, I’m wondering if anyone can help me with their thoughts on what carrier is best for my situation. My most commonly shipped box will be a 20×20x10 inch box weighing about 9lbs. It is fragile but not “Glass Fragile” more like “If you drop it on the cement sidewalk it will crack” fragile. – It will need fragile stickers if that even matters.

The shipping calculators on the carriers websites tell me that USPS Parcel post is the cheapest but I really have my doubts about USPS. FedEx Ground is just a little bit more but gets there in half the time. How much can you trust these calculators.

If anybody runs an online store I’m really interested in hearing what they use for their shipping solutions.