There are two more e-stores for online top-ups.
1.
ConnectPhone which has two additional advantages. The first is that there are more available amounts to top-up (5, 10, 15, 20 and 30€). The second, that there aren't any additional charges (if you use payment by credit card or bank deposit), while the Telesuite e-store charges an additional of 0.88€.
2.
Viva.gr which has the above advantages of ConnectPhone plus the advantage that is a very well known travel & VoIP online company. The disadvantage is that their 'mobile refill' is only available in Greek at the moment. However, it's quite easy to top-up, you just fill in your name, mobile number and e-mail to get your top-up code.