Add mine (been using CM for refills to our AT&T Gophone and T-Mo prepaid for almost 10 years, reliable & usually codes/fills to mobile numbers are done upon instant approval with Paypal or credit card payment) Maybe 2 or 3 times, held overnight due to technical issues/fraud prevention and needing CSR verification) They also do International Top-Ups, including Vietnam (Vinaphone's prepaid now, not before ^ )
http://www.callingmart.com/

ChinaMobile Hong Kong is our oversea prepaid provider, refilled online ourselves every 6 months or less as needed, with USA credit card (Verified by Visa and MasterCard Secure Code) instantly via their banking partner. As FYI, 1-Card-2-Number Prepaid Sim Card, good for Hong Kong, mainland China and for voice roaming at higher rates (AT&T and T-Mo) is what we use. https://www.hk.chinamobile.com/en/pr...ne-refill.html