Magento – Programatically resize and crop images

Add the following in your local.xml layout file (app/design/frontend/{interface}/{theme}/layout/local.xml) right under the <layout> tag this:
For product pages

<catalog_product_view>
    <reference name="head">
        <block type="core/text" name="some_name" as="some_name">
            <action method="setText">
                <text><![CDATA[YOUR TEXT GOES HERE]]></text>
            </action>
        </block>
    </reference>
</catalog_product_view>

For the checkout page is the same thing, except you need to replace the top tag with <checkout_onepage_index>. So the section above becomes:

<checkout_onepage_index>
    <reference name="head">
        <block type="core/text" name="some_name" as="some_name">
            <action method="setText">
                <text><![CDATA[YOUR TEXT GOES HERE]]></text>
            </action>
        </block>
    </reference>
</checkout_onepage_index>

it works the same for any other page. You just need to know the correct page handle (top tag). For index is cms_index_index, for cart is checkout_cart_index.

I would use magento form validation class, then create a new "remote validation", take a look at Magento form field AJAX validation

In your phtml file add

<script>

Validation.add('validate-customer-id', 'Please enter a valid id. For example xxx-yy-mmm.', function(v) {     

    // do your OMS webservice call in your controller 
    // you could use post this need to be secure
    var url = '<?php echo $this->getUrl('checkout/OnepageController/checkid', array('id' => 2345))?>' + encodeURIComponent(v);

    var result = false;

    new Ajax.Request(url, {
        method: 'get',
        asynchronous: false,
        onSuccess: function(transport) {
            var obj = response = eval('(' + transport.responseText + ')');

            if (obj.is_available === 0) {
                result = false;
            } else {
                result = true; /* return true or false */    
            }
        }
    })

    return result;
});

</script>

Update your input with

<input ... class=".. validate-customer-id" />

In your controller

public function checkidAction()
{
    $id = $this->getRequest()->getParam('id');
    if ($id=='1234')
    {
       $status = array('is_available' => 0);
    }
    else
    {
       $status = array('is_available' => 1);
    }

    echo json_encode($status);
}

Also see Magento prototype custom validation