quite a bit of hand coding going on in the gift certificate page that I think is contributing to the problem.
one issue i see is that the Add to Cart function on that page that is working reference the $ATC_itemdiscountable column, but on the gift certificate page, that column is not included in the functions, this will cause an error to occur and i think this is the root of the issue.
compare the add to cart functions on the 2 pages you sent and you will see the differences.