Hi MaryB,
When a customer checs out and opts to purchase shipping insurance, the cost of the policy will be adjusted to the correct amount based on our insurance rates chart and the end of auction value.
The "Additional Charge Amount" field is only to be used if you would like to charge more for insurance that our rates chart prescribes. For example, if you want to charge $1.50 for a policy in the 0-$50 range, you would enter 20 cents in that field.
If you have any further questions, feel free to ask.
-Mike