I am integrating a South African shipping company, MDS

I am integrating a South African shipping company, MDS Collivery with Marketpress (as per my client's request).

This shipping system requires that the suburb, town and province be identified as an ID from their system/database. Requiring users to type in exactly the right name (so that I can just do quick lookup is not going to work - what if the person make a n error).

My idea is to provide the user with a 'select' (containing the Provinces - my client will only ship locally in South Africa). After the user selected a Province, the user should filter by typing into the Town field (and then select the correct one from the list).
And the same again for the Suburb, thus eliminating most issues.

To implement it, I have to add the 'Collivery' class, which handles everything (this is done).

My Question is how do I enable AJAX for the shipping page (while the MarketPress get's initiated on the ajax calls - as I need the settings and the 'Collivery' class to do the lookups)?

Also, how do I overwrite existing Shipping Fields?

When this is done, I may release it for other people needing to make use of the Collivery shipping.