I make the same comment as payment methods. Availability of shipping modules is a make or break deciding factor when choosing an ecommerce solution. Probably more so than payment.
I believe it would be best to offer standalone and realtime.
Stand alone:
Flat based on order value or weight
Free based on order value
Table rates, as per 2.3
zones rates as per 2.3
Real Time
To be truly global the ideal is to offer a real time shipping module for the country’s government carrier. So for US, is it USPS, for Canada it is Canada Post etc. At least for the top 10 osCommerce country users. You’d need to research that.
Any other method that is supported by the osCommerce team should be in the box and anything in the box should be free.
For USA, people need FedEx and UPS and DHL.
For shipping it has to handle non-dimensional and dimensional shipping. It has to be able to configure the max weight per carrier. Freight has to be considered but anything that handles freight could be app store.
These are just my random thoughts on a Saturday morning 🙂