Which is the best shipping service for my store? USPS Vs UPS vs Fedex

I started a store in my town and I take orders online. I want to ship the products to the users all around the USA. Which is the best service to ship the items? I have been using USPS for a long time and they have good support as well in the form of lieblue.usps.gov
Please give your suggestions on which is the best shipping service.

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