Make sure you have a premium Mals-e account. If you don't, log in and click the upgrade link.
Then go to USPS and open an account. USPS sends you an email asking you to activate your account. Once you put your username and password into Mals for USPS, that URL they send you will be automated by Mals-e.
In Mals, Click on Global Settings under Advanced Setup.
Then click on Use Advanced Shipping Calculations.
Then click on Submit.
Then click on USPS Setup.
Click on the box that says "use quotes directly from USPS rates services" and put in your username, zip code and how you want the calculations (weight etc).
Next you have to click on Shipping Tables. Then create the zones for your calculator. Then when you go to add a product put the weight in where it says "Shipping Units". Put the weight in decimal format. For example, instead of 1/2, put in .5 etc etc...