The girls are relatively sheltered, which is why they didn't know about the suitcase trick that has caused the imprisonment of other young, religious Jews in foreign countries.
An update on Rivka's campaign page states the following:
UPDATE: 11/15: Thank you to all those who have contributed.
The Bulgarian court system has given us a deadline—if we can pay the necessary fees and reach our goal by THURSDAY, RIVKA WILL BE BEZ”H BE ABLE TO GO FREE AND COME BACK TO ISRAEL.
WE ARE SO CLOSE. PLEASE HELP US SAVE HER LIFE, WE HAVE COME TOO FAR TO GIVE UP NOW!!
The Zuaretz Family
B'ezrat Hashem, the goal will be reached in time.
Even if you already donated, please consider donating again if you possibly can.
Every donation matters.
On Rivka's page, you can see that some people donated as little as $2, yet altogether it still added up to over $138,000. (The goal is $200,000.)
Pidyon Shevuyim (redeeming a captive) is a tremendous mitzvah, and redeeming a captive girl takes precedence over a captive boy (halachically speaking).
This mitzvah reaps profound bracha to its participants.
Rivka and the other girls are really suffering in this prison. With the serious language barrier, plus the poor conditions & lack of religious accommodation, and the Jew-hatred from the other inmates (some of whom are awful, violent people), it's imperative to redeem these girls as soon as possible.
(And no, I don't know what's happening with Bat-El. Her page no longer shows the donation progress, so hopefully that means that they made it. But I don't know because there are no more updates either. Also, I don't know what's going on with the other girls in their group.)
UPDATE: Thank you very much to Hava who posted the name of Bat-El's mother in the comments! Please daven for Bat-El bat Inbal.
I still don't know specific names for davening for Rivka, so I've just been davening for Rivka bat Tali because that's the names of that appear on the campaign pages.
B'ezrat Hashem, they'll all be freed quickly & easily, and returned to their families in Eretz Yisrael.
P.S. If you know any young people travelling abroad, PLEASE warn them about these suitcase deceptions! This isn't the first time this has happened. The scoundrels who knowingly endanger these young, religious tourists by giving them a suitcase full of illegal stuff show no compassion or remorse for these young captives or their families.