Refund Shipping on Combined orders

[DingKeyDesigns]

I sell items which are on backorder

Customers will order 1 product and then order additional products before the first one ships

For example:

The first order they will pay $10 in shipping

The second order they pay $10 additional in shipping

When the first order is fulfilled, the second order is combined with the first one and the total for shipping comes to $15

The $5 difference should be automatically refunded back in the selling dashboard where the order was imported from

[DDD]

Can someone please help with this?? I use shopify with shipstation and would like an easier way to refund additional shipping costs when i combine orders. I have to go the shopfy and individually do it. Its a pain in the butt