Rather than a form, I would suggest using a link that calls an action to remove items from your cart. It is much simpler and there are less possible problems.
Template
<% loop $getCart %>
<% if $CartItems %>
<% loop $CartItems %>
$Title - $Amount - $Price - $Sum - <a href="{$Top.Link}remove/{$ID}">Remove from cart</a>
<% end_loop %>
<% else %>
Keine Artikel im Warenkorb
<% end_if %>
<% end_loop %>
Controller
private $allowed_actions = array(
'remove'
);
public function remove() {
$cartItemID = $this->request->param('ID');
if ($cartItemID && $cart = CartItem::get()->byID($cartItemID)) {
$cart->delete();
}
return array();
}