You must have "Visa approval letter" in your hand for picking up your visa-upon-arrival at the airport in Vietnam. Unless you already have valid Vietnam visa stamped on your passport. You cannot enter for traveling around Vietnam and will be forced to fly back your country of origin if you do not have the pre-approved letter for picking up your visa to Vietnam at the airport.
Vietnamese travel agents and companies who could help you arrange the pre-approved letter for picking up your visa-upon-arrival to Vietnam at the airport. "Service fee" as you may also see in the Service fee column is the fee that you must pay for arranging the "Visa approval letter" for picking up your visa-upon-arrival at any Vietnam airport by us.
Please note that the service fee does not cover the stamping fee, which will be paid in USD or VND cash directly at the airport when you pick up your Vietnam visa. In our case, we only require our clients to pay for our service if and thus, the term we call "Service Fee."