Giving the buyer a gift for purchasing over a certain amount

Q: A website is selling books at a discounted price. Whoever buys books for more than R250 gets a free bookmark. Whoever buys books for more than R750 gets another book free. Whoever buys books for more than R1499 gets free home delivery.

I want to know if transactions like these are valid where the seller gives a gift if the buyer buys products of a certain amount of money?

Seeking repentance for one's past sins before marriage

Q: If a man and woman commit zina before getting married. After their marriage, they had children and realized that what they have done before marriage was a sin. But they do not repent when being married because they dont have knowlegde that before marriage they had to repent.

After 6 years they got to know that they should have repented before nikaah, otherwise their nikaah is invalid. Is their nikaah valid or not as they repented after 6 years of marriage.

Taking a vow not to use Facebook and thereafter mistakenly opening someone else's Facebook

Q: One day my wife made a vow to Allah that she will never use Facebook in the future. Her sister uses facebook and always has automatic login into facebook on her mobile. When my  wife uses her sisters mobile, mistakenly she opened the facebook app and automatic logged into her sisters facebook account or profile. She saw her sisters Facebook profile picture etc. but she didn't use Facebook, she just saw it.