[ic] Shipping is calculated but not added on

[interchange-users@icdevgroup.org (John Allman)]

Hi - i have shipping calculated by a UserTag. What i'm seeing is that 
for certain users the price of the shipping is not added to the total on 
the checkout page. The shipping cost is calculated correctly and 
displayed on the checkout page but not added to the subtotal. The total 
is the same as the subtotal even though there is a shipping cost displayed.

Unfortunately our discount scheme is a little funny and i have a feeling 
it is at the heart of this problem somewhere... Has anyone seen 
behaviour like this before? if so could you tell me what caused it so i 
have a better chance of tracking this down? If needs be i can put up 
more specific details but i'm hoping someone has heard of something like 
this before? Using interchange 4.6.5

being driven slightly mad here...

thanks

John