Alternatively to setting up your own shipping rates, you can enable calculated shipping through USPS.
To install Calculated Shipping, please send an email to firstname.lastname@example.org, requested access to calculated shipping.
Once it is installed on your site, you will need to create a Calculated Shipping Profile. Navigate to Store > Shipping and click "Add a New Shipping Profile". Give it a name, description, and check "Calculated Shipping". Just like any other shipping profile, you will need to set the available destinations.
Once you have created your Calculated Shipping Profile, you can assign it to a product the same way as a normal product. However, you must also give your product a weight and size under the "Shipping" section of the product edit page. (Note: These fields only appear if a calculated shipping profile is chosen) The Store default for "Default product shipping type" is "Small Package", but if you are shipping your product in a envelope, you will want to change that. When shipping a product in an envelope, "Width" is the thickness of the envelope.
Calculated Shipping Settings
To configure your calculated shipping, navigate to Store > Shipping > Calculated shipping settings. Here, you can add your own modifiers to the shipping cost.