they aren't necessarily different words. both mean "forgive me," though they are used in different contexts. however, if you're talking "excuse me," then yes, they both mean it in the same way, but "извините" is the informal version.
I guess because "Извините" is more formal than "Извини", hence they want you to keep the formal structure throughout the sentence, which means using "вас" instead of "тебя". Though I don't think that your answer is necessarily wrong.
Извините у тебя есть яблоки (this is not wrong) duolingo needs to add this as an option. We don't know if the speaker knows the seller or not if the speaker does this is actually the more correct answer