How to get an invoice older that 18 months?

You can view your iTunes account's purchase history via your computer's iTunes and take a screenshot (iTunes doesn't issue duplicate emailed receipts) : See your purchase history in the iTunes Store on a Mac or PC - Apple Support

If you are referring to an Apple Store purchase then contact the store that you bought the item from and see if they can/will issue a duplicate receipt

Try contacting that online store and see if they can send you a duplicate receipt

********! You know you can't call apple stores. It puts you in an 800 number, you wait an hour, and they don't let you call an individual store. This is useless.

You can call Apple stores. Or do you not have your original receipt ?