No surcharges added for web refills. Refilled using a credit card off the web this week. The only additional charge I get is a a currency conversion charge (total of 3%) that Visa and my bank charges for any foreign currency transactions. 2 interesting changes: Now it's a direct add to your account, instead of having them emailing you a PIN number (they warn you to make sure you enter your phone number right!) and the lowest refill is now 10 euros instead of 15 euros that it used to be for Riiing/UM.

Yes, there are such more or less hidden surcharges but they are not on the UM's side - UM doesn't add anything to the total sum expressed in euros. I refilled in the first week of December by 10 euros and I had on my CC statement exactly 10 euros and its equivalent in Polish zlotys.

I think this way of refilling is better than dialing a long PIN via DTMF. IMHO PIN-based top-ups are convenient when using USSD codes (or when sending via SMS). Simply you can edit them before sending what is impossible with DTMF.