Corrected entry: Phoebe shows Ross' invitation to everyone, stating "Ross let Ben address mine." The envelope has poor kid like handwriting. The address is a 4 line address and the invitation also has stamps on it. When the camera comes back to Phoebe, she's holding an envelope with just her name on it, with no stamps. When she puts down the envelope by her side, the 4 line address is back. (00:11:30)Makes Sense
Corrected entry: When the gang minus Rachel receives their invitations, Phoebe states that Ross let Ben address hers, and shows the rest of them. We see that the envelope has a full address consisting of four lines. Chandler then enters, and about 10 seconds later we see Phoebe's invitation again, this time only having an address consisting of one line.