Ok, thank you, I changed flat.php the way you suggested.

Now I'm not getting the internal error when I check out, but in the shopping cart and when I click checkout, I get this error:

1054 Unknown column 'products' in 'field list'
in:
[select products-weight as check_it from products where products_id='2' limit 1]

And depending on which product I have in the cart, the number after products_id is different.

Does this have anything to do with the shipping modules??