Adding file size limit to zipcart

Hi and sorry my poor english!

I'm trying to change the zipcart in order to limit the maximum file size that one user can create. I worked two days trying to understand the hierarchy & functionallity of cart and zipcart, and now I have one approach that seems to work (but modifying both cart and zipcart, and withowt admin variables...)

3 questions:

1. I don't understand how zipcart can 'advise' to cart withowt change the cart module code. I tried to use the 'error' variable at cart template (by a local copy) but the ModifyCart.inc doesn't use it at any point... (It's possible that my problem was only not to domine PHP...)

2. My approach works fine when I reached the maximum amount of zip file size, and return to the cart withouth empty it and showing one error, but when all goes fine, when the download finalize, the gallery shows the cart without change (and really it's empty: if I do navegator's refresh show a empty cart) I doesn't change the zipcart code when the file size is good... where is my problem?

3. I'm very confused about the gallery forum's, developer's forum's, sourceforge forum's... If I want to colaborate to add new functionalities to zipcart, where I have to go?

Thanks!!!!