Well, pvp is not exactly a finished feature. Those cards are still "coming soon". I assume, you will be able to buy these boost items. After that they will want to balance it in a way that you face opponents roughly your own strength more often than not, so you have a need to use these items. The little bonus these give is laughable if you are not matched against a girl your own strength. Matchmaking is still a mystery to me. You very, very roughly get weaker opponents in lower ranks.
At least you can get presents for girls not currently unlocked. The event will go on for some days, maybe they will boost her stamina halfway through. The other girl seems to be buyable, maybe she can be unlocked in other ways during the event and has higher stamina.