This site will help you figure out the lowest prices and fastest shipping options.
It's a USPS, FedEx and UPS shipping calculator all in one. For free.

Head to the home page to get a quick estimate of shipping costs from multiple cariers.

• Start typing your origin ZIP code or city name in the "From City" field. Select from the autocomplete options.
  
• Do the same for the "To City" field. This field is for the destination city of the parcel.
• After filling out the cities, enter in the length, height, width, and weight of your package. The default units can be changed with the dropdown menus.
• Click the "Get Rates" button.
  The button will only be active when you fill out all the fields. It can take about 10 seconds or more to calculate and retrieve the shipping rates.

Discounts this site uses in its price quotes:

• USPS: This site returns Commercial Prices. You can get Commercial USPS prices from accounts on Shippo, Paypal, Stamps.com, Endicia.
• UPS: This Shippo discount is used.
• FedEx: This discount is used.

You can easily get discount rates for most carriers, even if you don't ship in large quantities. They generally require you to open up an account online, some charge a monthly fee to get a discount, but there are a few options that are free! (Well, you still have to pay for the postage, you just pay less!). See the shipping discounts guide for more tips. While you can guess weights and dimensions when you are trying to get rough shipping estimates, you'll need to be sure to use exact measurements when actually purchasing a shipping label. Invest in a scale and measuring tape so that you have the exact weight and dimensions of your package so that you can easily take advantage of buying your postage online at a discount. If you don't provide accurate measurements, your package will either be sent back to you, or you may end up getting charged a fine, so don't skip the step of providing proper dimensions and weights.