Hellou.
I'm pprogrammer and front end developer with 13yrs of experience, specialised in osCommerce stores.
For six years i was lead developer in computer store, based on osCommerce.
It was integrated with dozen of wholesellers and dropshippers so we had to deal not only with configuring store shipping calculations, but also include suppliers complex estimations.
All that should help me properly imlement your task and assure of its oscommerce coding standard.
If you feel the need of knowing more, I'm here to answear all your questions.
All the best,
Trygon eXtreme