You will want to put the full amount of your shipment in the Declared Value box to get the appropriate amount of insurance. With that in mind, there are certain USPS services that include liability coverage for free. We automatically take into consideration when calculating a Shipsurance quote for you.
For Commercial Base pricing, which is what Stamps.com offers, you will automatically get $50 coverage on your Priority shipments and $100 on your Priority Express shipments. Many of Stamps.com's Priority Services (Flat Rate, and weight-based up to 5 pounds) qualify for Commercial Plus Pricing-- for those you get will get $100 coverage included on both Priority and Priority Express.