What is required to get a visa-on-arrival for the HCM airport through an online service?
Is an onward air ticket necessary?
Is a minimum amount of funds needed?
Do you only give letter, passport and application at the airport?
TIA 

order to enter Vietnam, you will need a valid passport with at least six months validity remaining and a Vietnamese visa, a visa exemption document, or a written approval letter for a Vietnam visa upon arrival (Vietnam VOA). To obtain a written approval letter to enter, you must contact a travel agency prior to departure.
On the agent's web site or by email you have to provide the following information: 

- Your full name (exact used in the passport, so surname first)
- Present nationality
- Date of birth
- Passport number
- Passport expiry date
- Proposed date of arrival (the visa will start from this date regardless of your actual arrival date - you may enter at a later date but not before, so be sure to take time zone differences into account)

You printed out the approval letters and filled out the application forms before leaving home. On arrival in Hanoi, HCMC or Danang, you are requested to hand over passports, letters, applications and pictures. A few minutes and $50USD each later you had your visas. If you arrive without the application form or picture you can get them there.