Philip,

From what Giles has said it looks as though it is quite a big (and messy) job to complete this work as you need to decouple the address selection from the checkout provider and make sure that the content of each address is available before calculating delivery costs (which may involve moving the timings of these processes around).

The easiest way to get a handle on the code paths is to run the code through the debugger, putting break points at the key points (such as where the address gets entered, read or where a call to the shipping provoder is make).

If you are unable to do this, or if you come to the conclusion that "properly" refactoring the code is too complex/time consuming, then you might be able to apply a quick fix using session variables (set the addresses in the session when they are entered and read them from there when they are needed by your shipping provider?).