The least expensive service from USPS would be "First-Class Mail International Package". That service is priced by weight, in 4-ounce increments. If I read it correctly, one could send something weighing 2-1/2 lb for $19.45, assuming it meets the dimensional requirements for that service. No insurance, tracking, proof of delivery, etc. for that price.

Delivery time, according to the USPS, "varies". I don't know what that means, in practical terms, for a shipment to Singapore.