You don't need to change the method of payment.
If she pays by paypal then it is automatically marked as paid. She will have buyer protection.
If she pays by any other means it is not automatically marked as paid (You can mark it as payment received) and she will have no buyer protection.

If she wants it sent normal just send through another invoice with the discounted price. The buyer will then select the method. Perhaps just message her make sure she realises the discounted price is for DD only