Hi,
In the parameters of the onShippingDisplay method, you have the parameters $methods and $usable_methods
onShippingDisplay(&$order,&$methods,&$usable_methods,&$messages)
In $methods you have all the shipping methods prefiltered by HikaShop.
In $usable_methods you have an empty array.
So what you need to do is to make a copy of your shipping method data from $methods, overwrite the shipping_price with the fee retrieved from your shipping company and then add that copy in $usable_methods so that HikaShop can use it as an available shipping method on the checkout.