George - Send them in a flat rate Priorty mail envelope, the cost should be $3.95 and charge the exact price for insurance which is $1.30 for the first $50, $2.20 for up to $100.00, $3.20 for up to $200 coverage etc.
You can go into the post office web page and get the totals by following the links.
Post office web site www.usps.com.
Walt