They opened the envelope and found this:

Mr Avocado knew it was a QR code and googled how to decipher it.

They downloaded the application and finally got the instructions:

Go to the city big square and get into the church.

They entered and looked for clues. After a long time, they found a mistery room at the end of a dark corridor. They got into the room and saw a priest… he had the glasses!!!

The priest ran away but he left a misterious book on the table with a note:

GO TO THE LIBRARY.