special shipment extension

Hi All,

I am new to Opencart, I started with version 1.5. I need a shipping logic, where the shipping address is not given by the customer, but there are pickup points what he/she can choose from. The cost of the shipping depends on the selected pickup point.
I browsed through the code to check how can I develop such a shipping extension but I realized that normally it is not possible without modifying the core files.
Am I wrong? Is there any solution to my requirements?

Regards

You can do this if you create your own shipping option which won't need to modify any core files

I tried to create a shipping modul but the checkout always asks for shipping address. Is there any normal way to skip a step in the checkout flow? e.g asking for shipping address