correct, the base rate is set by MasterCard and can be obtained using the website that was posted earlier.
banks and merchants can of course add on any type of fee they deem fit and they don't necessarily have to disclose the exact percentage. or they can apply a surcharge that is higher than the merchant fee charged to them by MasterCard. so the effective rate can be much less favourable.
however, in my recent experience of using an Australian-issued VISA debit card on Lazada, no additional fees were added by the merchant or Lazada.
the only difficulty I had was that some local merchants don't accept foreign cards, presumably because the merchant fee is much higher (MC can take 3% or higher).