Best for you is the paperpass then :)

Then you have a paper with a price and the traveldiary to show where you traveled with it.

Thanks! But I need the e-ticket because I probably will have to buy it only the day before my travel. Properly, I’m wondering if on the app, it’s possible to have a page with the price of the ticket and the first and last cities with their travel date. 

You will get a payment confirmation with the price paid. That's right after ordering, so without any travel details.

After the trip, you can export “My Trip” as an image and as a link. The image may be useful for you: it contains all journeys you've made, complete with departure/arrival stations and departure/arrival times.  The link is a URL to a map of Europe with a graphical representation of your trip.

You can even buy the paperpass on many stations across Europe (for direct use) note depending on the Route you have to pay for Reservations on top of the Pass :/
 

The poster is from Belgium and Belgian railways have decided, in their infinite wisdom, that it's better to limit their sales points for Interrail to exactly 0001.

>Warning: may contain traces of irony.]

No online order shops in BE like f.e. treinreiswinkel in NL? Allthough I assume they can even send out to BE-the postage will be a little higher. I assume this is because they need to have a special printer, different from the ones that sell local NMBS tickets

Note that a paper IR is a normal printed INTernational ticket, with the dates from/till and your name+ID code on, is is attached to the so called Travel report which is A4 and folded twice, on this you have to write down all trips. The ticket as such has also fields for the traveldays (if you have that format). Hence it does not tell to/from as normal INTern. tickets on that antique format do..

As an aside: I really wonder now how long these paper passes will still be available?! F.e. NS in NL (as I read on their own forum) does only sell paper passes at 2 locations, not even in their own main office in Utrecht-where the organisation sits.

Nope. But the poster wants to buy it one day before travel so online order of a paper ticket is not an option. Buying at Brussel-Zuid could be an option, if Lea lives in the neighbourhood.