Changing the address for tax calculation in order-payment page

[supervipersly]

Hi

I wrote a module to allow customers to order products and select a pickup point in another city. It all works fine until I have a customer from one province (we are in Canada) who selects to pickup his order in another province.

In the order(class) I set up the iddeliveryaddress to that of the drop point when the order is complete not problem, but I cannot set the iddeliveryaddress for cart object in order to get proper tax calculations (we charge tax based on delivery address).

Any help or do you know of a module that does this properly so can I buy and and implement it in mine (not for resale).

Thank you in advance

Sylvain