You can write Brianne for free, while she is in the MTC  in Provo, UT, by going to:  
Then go to:
"write a letter"
Scroll down to PROVO MTC  
(I think it is the second item)

Her box number is: 83
Her mission code is: ADR-TCN
Her departure date is: July 29

Packages sent to the MTC should be addressed as follows:

Sister Brianne Michelle Peterson
Adriatic North Mission
Provo MTC
July 29 ADR-TCN
2007 North 900 East  Unit 83
Provo, Utah

Please remember her birthday is June 22 - She would love birthday cards ;)