To configure a default printer for use with USPS(without postage) and / or to set up a default label location, go to the Shipping menu in Shiplark and click on Configure Shipping Providers. Then click on the shipping provider, for which you want to configure settings. The same settings can also be accessed by going to File >> Configure store >> Shipping, and then clicking on the relevant shipping provider.
To set the default shipping options with USPS (without postage), go to Shipping >> Configure Shipping Providers >> USPS. Once there you can set the printer defaults as required and then click on Configure Default Shipping Rule to access the default settings for USPS (without postage).
The default settings allows you to configure all the settings you wish to use as default. Incase you use International Shipping, remember to set defaults for that as well.