You should buy enough insurance to cover the full value of the shipment.
While UPS shipments automatically include $100 of UPS insurance, ShippingEasy does not account for the included carrier insurance on the package when additional insurance from Shipsurance is purchased. Instead, insurance will be purchased for the full "insure to value" requested.
The greatest benefit of this is that you will not need to file the claim with two providers. You will only need to file with Shipsurance if anything were to happen to the package, ensuring that you will be reimbursed as quickly as possible.
What is UPS Declared Value?
How much does your insurance cost?
How does billing work for Shipsurance?
How to: Insure a package
How to: Find out how much coverage was purchased for a shipment from Shipsurance