just pay telco bills via their respective website payment portal
I don't think that works since we're talking about zeroing out NETS FlashPay cards. However, I think SP Services might accept NETS FlashPay at their own service counters and possibly also at 7-Eleven stores and post offices. If that's correct, they would let you pay any precise amount you wish to drain the remaining balance from your NETS FlashPay card. That'd work well. You still have to pay the rest of your bill, of course.
Make sure you shut off Automatic Top Up (ATU) on those NETS FlashPay cards before you try to drain them.
Upon reflection, if you find a shop or supermarket that sells bulk foods by weight -- there are such shops in Singapore -- then you can buy precisely the right amount of bulk food to equal any specific monetary amount. That'll work better than bananas at Giant. Bulk rice would work really well, but even bulk peanuts or coffee beans should get the job done. Tell the shopkeeper you want precisely $8.32 worth of rice (or whatever the amount), or weigh it yourself if allowed, and there you go.
If the Post Office accepts NETS FlashPay, how about buying some postage stamps? You can definitely buy postage stamps in 30 cent increments (20 gram local letter rate), but I think the Post Office will sell you smaller denomination stamps, too. So you should be able to get a very precise amount this way. Then just hang onto the stamps until you want to mail a letter. If you only have 15 cents worth of stamps left, and need to mail a 30 cent letter, then just stop by the Post Office again to buy "top up" postage.
