If the shipping insurance and shipment method do not match, you may not receive insurance.
Priority Mail Box shipments automatically receive free shipping insurance up to $50 (no need to check the box to receive insurance).
Shipping Insurance - Priority Mail Box
Add Insurance ($0.00)
Add Signature Confirmation ($2.35)
If you are unsure which insurance option to choose, Estimate Shipping & Taxes then only the correct insurance option will be displayed.
Checking "Priority Mail Box Insurance" when you choose the US Envelope Shipping method will not get you free insurance.