Update item quantity in a shopping cart via a textbox

Yup, I know this is a common question that has been asked before; not just here, but in other forums and tutorials I've also checked. I just can't get it to work.
Currently I display my shopping cart with the following GridView, fed by a DataTable in the code-behind:
<asp:GridView ID="shopGrid" runat="server" AutoGenerateColumns="false" GridLines="None"
DataKeyNames="id" CssClass="shoppingcart">
    <Columns>
        <asp:TemplateField HeaderText="Quantity">
            <ItemTemplate>
                <asp:TextBox ID="item_qty" runat="server" Text='<%# Bind("qty") %>' AutoPostBack="true" />
  &nbs ...