zip code not registering

[ozysean]

I am having a problem with the cart where when someone puts in their zip code and then changes the type of postage it does not keep the zip code resulting in an error.

 

You can see this on my site www.luthierssupplies.com.au , if you add an item to the cart and put in a postcode, say 2000, click update postage it brings up the total with postage tax etc, fine. But then when you click on the other type of postage it does not keep work as the zip code gets lost somehow.

 

I have the Ship in Cart contribution installed and I think it has something to do with the tep_session_register('cart_zip_code'); line in this, but I don't know how. Basically the site needs to 'remember' the zip code.

 

Any help or suggestions would be greatly appreciated.

 

Thanks in advance

 

P.S I have no idea what I'm doing with php, it's really hit and miss with me, I'm hopeing someone out there has the knowlage to solve this without wasteing anyones time