I'm currently part of a group of scouts in Delft and we (12 aged 15 through 17 & 2 over 18s) and we are trying to get to Amsterdam as cheaply as possible. We have looked at group tickets (https://www.ns.nl/producten/en/losse-kaartjes/p/ns-groepsretour) but these appear to need printing so aren't practical (we are camping and don't have a printer) we have some mobile phones (not enough for 1 each - would it be possible to show the tickets on these?).
However, I then found a link to what is described as a promo day ticket at 12.50EUR (https://www.ns.nl/producten/en/s/dagkaart-juli-1606072) but I'm struggling to find any information, where are these tickets valid for and is there any time restrictions? And would we need to print off something to use them?
Apologies for the formatting as we are sending this from a mobile over dodgy WiFi. Huge thanks to anyone who can fix it.
Edit: quick additional question, what is the ideal payment and it used with a UK credit card?