There is a setting in admin shipping method to set Additional Handling Fee and markup like the Folloiwng PIC.
How to Post shipment to sendle
There are 2 ways to post it to the sendle. 1) Post it at the order submit Or 2) Post it at the shipment creation in admin. There is a configuration in sendle shipping method to set it. You can see “Book shipment On” with 2 options “Order Submit” and “Shipment submit” in the sendle shippimg method settings.
How to find Sendle shipping method setting in magento admin panel
Go to admin -> top menu -> System -> configuration-> left tab -> sales -> Shipping Methods, like the following PIC