Will a shoplifting as a minor be removed at 18 and what will happen if I don’t receive something in the mail?

Charges that a person receives in juvenile court are sealed from the public's view. So, if you were charged as a juvenile, the disposition of those cases are sealed. You don't indicate whether the '2nd time in trouble' was a juvenile case or not, so I can't address this issue. But if there is a 2nd offense all you have to do is call the clerk of the court and find out where the case is at this time.

If the police were not called and you didn't receive a ticket and no court date, then it sounds like the store didn't call the police and just had you pay their civil demand while you were at the store. If so, then nothing will show up on your criminal record. Keep in mind that in the eyes of the court system, you are an adult at age 17. If you know where this happened (what county or city), you can call the municipal court and ask if they have issued a warrant for your arrest or if you have any charges that have been filed against you.

If a court gave you a fine, then you will have a record. No expungement is available. If "they" is the store from whom you stole, then you will not have a record.