choose any of the actions available/included in the order form template and just output yoru checkbox (or whatever)

probably with attribute required.
something like this i would have thought

add_action('wppizza_order_form_before_profile_update','my_checkbox_stuff');
function my_checkbox_stuff(){
print"<label>some text<input type='checkbox' id='some_id' name='some_name' value='1' required style='float:left'/></label>";
}