For the last five years I've paid road tax on my 50cc bike - €3 + €12 = €15. Each time, I've enlisted the help of others but this year I'd like to do it myself.
The tax for 50cc vehicles is due in April (unlike other vehicles).
The method is to locate the relevant webpage, fill out the form, print out the payment sheet and take it to the police for checking.
If it's OK, take it to the bank, make the payments (plus the bank's commission of about €5), sign the sheets then return to the police with the documents for rubber stamping!
The website is https://www.gsis.gr
Click on e-παράβολο
to access the form.
For the sake of others, I'll detail it here:
In the left-hand column click on
Είσοδος για μη Πιστοποιημένους Χρήστες στο TAXISnet (Login for Non-Certified Users to TAXISnet)
On the next page click the button at the bottom (Χορήγηση Παραβόλου)
Now, select from the drop-down lists as follows:
Δημόσιας Τάξης Π.τ.Π./Ελληνική Αστθνομία
Άδειες (Ένσημο ΕΛ.ΑΣ.)
 Άδεια ή ανανέωση άδειας(έκδοση από ΕΛΑΣ)
When you've filled out all the fields (good luck with that)*, click the bottom-right
button (Υποβολή Αιτήματος)
Do NOT click the left button (Clear Form)!
Note: although I was told to do this at the police station in Haniá, I was able to do it at Vrises police station this year (April 2019). It depends on your locality. Also note that some lesser banks don't do 50cc tax processing.
* If anyone has the time and inclination to translate the form fields, I'm sure that someone (I for one) will be very grateful in April!