cross reference values

cross reference values

I have created an app for inventory management for my online sales. I have created a drop down menu for shipping options. My goal is to select the ship type in field1 and automatically populate the cost for that type shipping in field2- rather than having to manually type it in each time. I would also like the option to choose "other" as shipping type and enter my own value if needed. And if shipping charges change, I can change it at the data source...

How should I be doing this? As a form within my app?