What does 'pp' at the end of a letter mean?

Answer 1

It normally precedes the name of the sender and means per procurationem or on behalf of. It would be used if the letter was signed by a secretary or assistant and not the person in whose name the letter was sent. eg signed: Molly Smith pp the Branch Manager

Answer 2

pp stands for per procurationem.
that's the easy part
unfortunately the Latin appears to mean either:

"through the agency of" more accurately/literally " through the care/caring of"

or

"on behalf of"

however, the second of these appears to come from a mis-translation of "per pro" as "through and for" in the sense that "pro" means "for" as in "pros" and "cons" - "fors" and "againsts"

so the solution to where to put the pp depends on what you think it means.
one source suggests that "on behalf of " is a wrong translation.