Wife was recently in Singapore and did topup of Nets FlashPay (NFP) card using GTM machine with Apple Pay and US Bank Altitude card. While the transaction was successful (before this moment topup of transit cards with foreign cards were disabled in Jan 2020), seems now Translink adds 3.5% transaction fee that is bundled together as a single charge.

So if you are planning to use MRT/buses a lot in Singapore and have foreign issued card, your options are either eat that 3.5% fee and use the NFP or use the card (or apple/google pay) directly and have 50c added daily to your fares. I'd say the first option is better.