Depends how their system is setup, so hard to say, especially with the unusual fallback feature of this card. It could be that when they sell a money order, it puts the terminal in "debit mode" so it will automatically only work with PIN debit cards, and thus kick the UnionPay card to fallback to debit mode.

I didn't include this data point, but I tested an Arco chip-capable card terminal inside the gas station building (note: where Arco gas is cash & PIN debit only), & it acted like Grocery Outlet where it rejected the chip, asked for swipe & then required a PIN. Now, this is still different because the Arco terminal is not setup to accept the PBOC chip on this card, whereas USPS is setup for that chip type.

However, there are many possibilities of thousands of configuration combinations between the terminal, terminal software, backend hardware, processor, card behavior, & more that the only way to really know & discover loopholes & flaws in the setup is by testing a real transaction.

Maybe you can buy a small money order as a test & report back.