Translation:I have only three packages that I have to put in the mail.
Why is posxton in the accusitive? shoudln´t it be either ´en la posxto´ or just ´la posxton´ Helpu!
Because you are not in the mail when you are putting them (location), but when you put them, you are moving them from your possession to the inside of the mail system (movement).
"En" takes the nominative when expressing location but the accusative when expressing movement.
Another way of looking at this is that the n-ending can take the place of a missing preposition. Take this example from the autodidactic book Saluton! by Audrey Childs-Mee:
- Li metas la bildkarton al sur la tablo = Li metas la bildkarton sur la tablon.