Stamping fee is a fee which direct payment to Vietnam Immigration Officer at the arrival Airport to get your visa stamp. This is fix amount to pay for Goverment.
Cash is recommended to prepare before you arrive at any Vietnamese airport:
$45 USD for single entry visa, from 1 to 3 months single visa
65 USD for 1 month multiple entry visa (< 30 days)
95 USD for 3 month multiple entry visa
For convenient reason, you prepare cash to pay stamping fee before entering Vietnamese airports because you cannot pay by credit card, in case that there is not International ATM in the airport.