Please note that we are not Vietnam Embassy / Consulate or Government site. We are non-government website run by a private company. However, we do work legally under Vietnam business law and Vietnamese immigration law in order to arrange approval letter at Vietnam Immigration Department. Accordingly, Vietnam visa on arrival at airports is legitimate & supported by Vietnam Immigration Department. 
We must establish legally under the laws of Vietnam & we must provide paperwork to the Vietnam Immigration Department in order to get approval letter for your visa on arrival at Vietnam International Airports. Therefore we have to charge you a fee that we called service fee. Apart from that, you must pay for stamping fee in cash at Vietnam Airport to collect your stamped visa. It’s $25USD/person for single entry visa of 1 month or 3 months; $50USD/person for multiple entries visa of 1 month or 3 months.