Seat reservation Barcelona-Valence ''not delivered on time''

Hello!

I want to make a seat reservation for train from Barcelona to Valence. I tried it through interrail website and it says that is has to be delivered to me by post and also that it would´t arrive on time (but my travel is in august and now is june, so i don´t get why it would take so long to deliver it). I know it has to be on paper and not online, but i don´t know what to do as long as i don´t want to buy it that day at the station because i´m scared it would be sold out. Isn´t there any way shipping could be faster? I also tried making the reservation on SNCF website, but there is no way how to do that there if i understant it correctly. Thank you for any help!