Thanks!
That solved the problem.
I've read through the uploadproduct.tmpl and found
if ( $Globals['zipcode'] == "yes" ) {
at the top, in the javascript section.
Wouldn't the same code before this section solve the "problem"?:
<tr>
<td class="{$Style['tddetails']}">{$Globals['pp_lang']['enterzip']}</font></td>
<td class="{$Style['tddetails']}"><input type="text" size="5" name="myzipcode" /></td>
</tr>
<tr>