Train ticket payment receipt good enough for 144-hr visa waiver?

Hi,

We want to use the 144-hr Twov in Shanghai and after that go by train to Hong Kong. However, train tickets to Hong Kong are only delivered to addresses in China, so we cannot show it at the Immigration at the border in the Shanghai airport. We Will of course have a payment receipt by then. Is it good enough to show the payment receipt as a proof of having a confirmed onward ticket to a third country for the Twov?

Ivo