Since the WorldPay checkout is a remote checkout you should only send out a receipt when you can confirm the transaction is complete. This would be done on your return-POST or IPN page that you have. When you receive the post back from WorldPay and confirm it is successful you can then send out an email receipt with Universal Email.
If you have not implemented this return-POST page yet please refer to the eCart help for more information. In the help this is located under Using eCart > Configuring Checkout Support > Gateway Specific Information > Remote Checkout > WorldPay.