Croydon Tramlink with railcard

I'm hoping someone can help explain some stuff to me:

I need to travel from Guildford to New Addington during the morning peak and return at some point later that day. I have a young person railcard and I'm confused about what the cheapest fare is.

I haven't seen the through ticket to the tramlink for sale on my usual ticketing website but it is on brfares.com at £18.90 without a railcard. Apparently there is no fare with a railcard, is this right?

A RTN to Wimbledon costs £12 or £12.50 (min. fare is £12) and with two tram fares that would total £16.60.

An all zone travelcard which I think is valid on trams would cost £19.80

So basically I'd like to know is railcards are actually valid on through tickets for the tramlink to reduce that price I quoted. And also are these actually available from the ticket machines or office in Guildford.

I'll be claiming the journey on expenses so it doesn't really matter at all if I get the very cheapest, I'd just like to know.

Thanks in advance