Make sure you get guaranteed payment first. Then I would suggest a NAME shipper like UPS, DHL or FedEx. Insure it (charge buyer for insurance too). Not that the Singapore buyer would try to scam you, but there are many scams that come from that area. USPS would probably not want to handle a keyboard sized package, but I always check with them for smaller items. They may take a day longer, but they're much less expensive.

Good luck.