iPayment

We have iPayment activated using Authorize.net. When we accept a web order (using SAP DI) we authorize CC for the anticipated value for products and shipping. On add Delivery if the original authorized amount was changed due to product add-ons, etc. when the delivery screen iPayment popup appears the correct updated amount is properly displayed. However, on submit instead of re-authorizing the authorization issued when the order was placed a new (2nd) authorization is created and Authorized (Pending/Capture) is then ultimately Captured (Pending/Settlement). However, our customer still has the initial authorization still placing a hold for the original amount against their CC which should have been reauthorized for the revised amount or dropped when the 2nd authorization was issued. Strangely, if the initial authorization was for $0.00 that authorization reauthorized with updated sales value and ultimately captured on copy to Invoice. How can we fix this??