You can see the discounted fare for yourself on www.superfast.com or www.anek.gr.

There is more as 1 ferry, price is also season dependent and of course what type of accomm you want to use and DO note there are far more ferries, also for shorter crossing from Brindisi or rather longer from Ancona. Many seem to think one has to use those for the discount-but about all give discounts to ju/seniors and the final price to pay may even be lower as on those from EUrail.

Due to sky-high current fuel prices all charge extra for fuel-this will never be discounted.

I will be travelling from Igoumenitsa to Bari later this month; my fare was reduced from €72 to €27, but having contacted Interrail I can confirm that what you do pay is indeed considered only fuel charges, port taxes, etc; and must, therefore, use up a travel day.

I thought this seemed fair enough, given the €45 saved and the free airseat with my 1st class pass. It is perhaps a bit galling that it is being sold as a “free” crossing when it isn’t really, but it’s not really any different from some of the pricier international high-speed trains in that respect.

I did Patras to Bari last week on Superfast and the price was reduced to from €72 to €27. It was about €20 more to go to Ancona.

It did use up a travel day as you need to activate the pass. However on the way down there (Bari to Igoumenitsa) , I’d used up my 4 days but because the pass was within the one month period, Superfast accepted it and gave me the same discount.

Thank you all for those answers. 
Can you confirm if you book last minut or if you did it in advance ? 

Did they say that?

IMO that's justified because with the surcharges they've invented for pass holders only, you effectively only have a discount.