is obtaining a visa without a travel agency feasible?

I think the days of having the visa on arrival rejected are in the past. I may be wrong but I've not heard of anyone having issues - and what's more, the partners I work with in Iran have no issue with our clients getting a visa on arrival - if they thought there was a risk they'd strongly recommend getting a visa beforehand.
If you get the visa on arrival you don't need to bother with getting the authorisation number - that will happen at the airport. Getting the authorisation number will probably make things faster at the airport but is not necessary - I actually had a number, ended up not giving it to them. No issues. I also know of various people who got the visa on arrival without the authorisation number.

isn't the VOA only for 14 days though? we were planning on a longer vacation, and I guess getting the visa duration prolonged would be hard (i really don't want to spend a day in a police office in Tehran with 40° hoping our visa will be prolonged).
The Milan Embassy strongly advised getting a visa before leaving, but the tales i read online (including yours) did reassure me of my chances to get a VOA...

The VOA is now 30 days. - though I think it does depend on how long you say you're there, So I told them I was there 16 days and got a 21 day visa

do you have official sources for that?

I also read it here and there (wikitravels, lonely planet...), but our embassy explicitly told us the VOA is 14 days, and that if we wanted more we needed to get it extended (up till 90 days) once already in the country (with no guarantees of course).

Well I don't have an official source regarding Italians it may different, just came across this article which implies relations may not be as good with Italy as other countries? (http://english.almanar.com.lb/adetails.php?eid=268816&frid=19&seccatid=32&cid=19&fromval=1) after all the visa rules are different for UK, US, Canada, Colombia and some others. Maybe try and find an Italian travel forum to get latest info.

Hello, i'm a 20-year old Italian student planning to visit Iran this july.
As such, my budget is very limited, and i would like not to spend extra €40 in a travel agency obtaining a voucher. I read that it is possible to obtain a visa by having the hotel one spends the first night in contacting the Foreign affairs ministry: is this true? would anyone know of one such hotel in Qom/Tehran, willing to do this

The hotel doesn't contact anyone. They simply need to send you a confirmation by email (this must include the hotel's phone number as it seems that the visa office at the airport may phone the hotel to confirm the booking). All I had was a print out from Gmail. Visa process took 30 minutes from start to end and cost €60 (you make the payment at the bank next to the visa office in the immigration arrivals hall). I stayed at Hotel Merkasi (also called Hotel Central). Good value and very good location and great service. They will also arrange a car at the airport for about $25 to the hotel. Well worth it for a first time arrival.

The hotel doesn't contact anyone. They simply need to send you a confirmation by email (this must include the hotel's phone number as it seems that the visa office at the airport may phone the hotel to confirm the booking). All I had was a print out from Gmail. Visa process took 30 minutes from start to end and cost €60 (you make the payment at the bank next to the visa office in the immigration arrivals hall). I stayed at Hotel Merkasi (also called Hotel Central). Good value and very good location and great service. They will also arrange a car at the airport for about $25 to the hotel. Well worth it for a first time arrival.

I assume you are talking about visas on arrival, correct?

Hi
I entered two times iran by visa on arrival (croatian passport)

Dic.2016,Isfahan airport.
No code before.u just showed up.had an email from hotel and a travel insurance which mentioned valid in iran..i got a 10day visum(i wld stay 5days) in 20minutes.

May 2016...Shiraz airport.. Had all documents as above mentioned...they looked only at insurance... In 1h i got a 10day visa.(i wld stay 4days).attention in Shiraz airport there was only 1bank and it had no money (to change)

Okay guys, Thank you for all your answers.
I had a chat with a secretary of the Iranian embassy in Italy, who told me that invitations are a thing of the past, as are organized tours (except UK/canada/USA nationals), and are mostly used by tourist agencies for profits.

sooo i'm still going to get a visa before departure, but there should be no problems.