I don't understand why you think Auctiva is overcharging buyers for insurance?
The postal service rate has been the standard for years and that's what I've always charged, whether I purchased insurance through USPS or elsewhere. Many sellers do that.
I don't use Auctiva's checkout, so I could be wrong on this one, but doesn't it charge the insurance rate you choose? You could offer the buyers discounted insurance if it bothered you not to.