The cheapest way is when the person or organization you’re paying in U.S. dollars accepts a major credit card without any markup. Then you can use an ICBC Global Travel Mastercard (as one notable example) to make the payment and actually come out a bit ahead, effectively paying less than spot rate for U.S. dollars.
Interactive Brokers is the next least expensive. You’ll get spot rate and only pay the broker a US$2 commission, plus GST on the commission if it’s Interactive Brokers Singapore Pte. Ltd. But Interactive Brokers is not a payment service if that’s what you have in mind. They provide foreign currency and securities trading services.