Now the good side of this that they do not say in their t's and c's that this offer can not be used with any other.
"Terms and conditions:
* Promotional period begins at 12:01am on 22/06/2007 and ends at 11:59pm on 30/06/2007.
* In order to be eligible for this offer you must pay for your order via Paypal. Orders paid for by other methods are strictly not eligible. No exceptions.
* Limit of one gift voucher per customer.
* Gift voucher will be sent via email.
* The gift voucher will be sent out within 1 week after the promotion date. (07/07/2007)
* Gift voucher expiry date: 07/10/2007.
* Promotional period start date/time and finish date/time are strictly non negotiable. No exceptions.
* After paying via PayPal, your details will be automatically logged and the voucher sent separately."
So you should be able to get your $20 voucher, free shipping and your Buckscoop cashback