Hi! Welcome to our NS Community, vvventsi. Season tickets go per month, so not sure about the 20 days, but can you tell me on which days and what times you will be traveling at? You can find this on https://www.ns.nl/en.

A month may work if it's cheaper than the trains that I'd need to take. Probably I have to catch maybe 3 trains and if it's cheap enough I may go around the Netherlands for sightseeing. I'll have to travel on Tuesday and Thursday for sure. On arrival, I'll have to catch a train at 2:50 which arrives around 5:30. Last time, a lady from information told us we can buy a group ticket and that's what we did. I thought that we could only use it if our whole journey was during the off-peak hours but she told us that we can still use it. On return i wouldn't have to hurry to catch a flight as I'll be travelling around the Netherlands so i can catch something during the off peak hours too. It's possible to travel during the weekend too.

Depending on the route, my advice is to check the NS Flex Dal Voordeel or the NS Flex Weekend Vrij. If you only will travel during the weekends, then the NS Flex Weekend Vrij would probaly the cheapest option for you. You can also do the selection guide on the website.

I think tge NS Flex Dal Voordeel will suit me perfect but i didn't understand if i have to have OVP card?

Yes, you do need an OV-chipcard for that. If you have a Dutch IBAN, you can order the OV-card with the NS Flex Dal Voordeel. If you don't have a Dutch IBAN, you can order an OV-card on www.ov-chipkaart.nl and contact our customer service to order the contract when you have the card.

You need to apply for a personal chipcard (the subscription is personal) and than you can travel at 40% discount during offpeak hours by checking in/out at the gates. During peak hours you still pay full fare.

And they can ship it to Bulgaria?

I guess so; a quick search on the website gave this result:

Do you have a foreign IBAN but do you not live in the Netherlands, Belgium, Germany or Luxembourg? Please fill in our contact form and we will help you further. You can apply for an OV-chipkaart with any valid IBAN.

from: Apply for personal OV-chipkaart - OV-Chipkaart.nl

Good luck!

Good luck indeed because that contact form/page does nothing except telling me to accept cookies (which I did and then still nothing).

That page only works after allowing 3rd party cookies. Bad design :(

Update: 

I was going through the NS app and found 

Young person's Day ticket off-peak hours. I think it'll fit me the best as there's no subscription etc etc. And this time I won't have to hurry to catch the flight so i can make my timeline to go within off-peak hours. The only thing i didn't understand is that it says that the journey must be withing the off peak hours. Last time we should have gotten a train at 14:40 but it got cancelled so we got one in 15:20 and some women at information told us that we can get group ticket, no matter that we will have to ride during peak hours too

With a Young Person's Day Ticket Off-Peak Hours, or a Group Ticket, your entire journey needs to take place during off-peak hours. You may not be on a train during peak hours with this ticket. However, if you had planned for a journey that would take place during off-peak hours in its entirety and because of a cancelled train you now have to travel the last part during peak hours, then that is allowed.

So to be clear: 

You were right here and you were misinformed, unfortunately.