If the packages are heavy, then UPS or FedEx may be cheaper, otherwise the post office is probably best. If you have a scale and can weigh your packages, you can go online to usps.com, fedes.com or ups.com and get a free rate quote.
You will get better rates from UPS and FedEx if you make a free online account and print and pay for your label at home. The stores, charge a mark up for providing the service.
Typically, if the post office will cost over $10-$12, I use FedEx.