When you pay in SGD but the charge is done overseas, it will incur DCC charges. Not earned by the merchant, it's just a processing costs.
So when you go overseas shopping, they ask if you want to charge in local currency or SGD, ALWAYS pick local currency. Their SGD rate is already inflated and you get DCC charge on top as well, modern day "trick".
Whereas for PSN, the payment is not processed in Singapore, you will get that charge. Same goes for a lot of other merchants, such as Nike SG. Some portals using PayPal can avoid that charge because PayPal can process it locally, because the payment goes to PayPal Singapore.