How to find the legal, yet best priced authorized visa on arrival approval letter agency for Vietnam tourist visa

To visit Vietnam as a tourist, some must successfully apply for a Visa On Arrival (VOA) approval letter and carry such approval letter printed, both when arriving to departure airport and a destination airport (international airport) in Vietnam.

To try to get such approval letter, one has to visit a website of one of the Vietnamese government Authorized Visa on Arival Approval Letter Agency (AVoAALA), fill in a form and pay a fee;
For one month VOA approval letter, issuing fee will be about 18 USD.

If the try was successful, after about 5 days (or less, if an additional fee payed), an approval letter will be sent to the email given in in the form.

• When reaching to a departure airport, checking-in in the flight company's counter must be done with this approval letter (their worker might not care if it is printed or from smartphone app but a Vietnamese border officer probably will).

• After I landed on Vietnam, I went tot the VOA office and filled in a required form, I then gave that form, the printed approval letter and my passport to a Vietnamese border officer which examined all of these and then demanded me 15 USD or 25 USD (I don't recall exactly) stamping fee for stamping the visa that should be prepared in a few minutes after payment;
  I payed and waited for the preparation of the visa and later when my passport was returned to me with that visa, I could enter Vietnam via border police counter.

My problem

I understand that there are many AVoAALA and each one can charge different prices (either per month of stay and/or other reasons).

To work simple and fast I chose the AVoAALA recommended by the Vietnamese embassy of the country that a citizenship of which I currently hold, but I recognize no must to handle like this if I decide to go to Vietnam with a tourist visa again.

My question

How to find the legal, yet best priced authorized visa on arrival approval letter agency for Vietnam tourist visa?
_{Perhaps there is a list somewhere - or a search engine query trick that could help.}