In xml Invoice Standard v2.1, Value for MailCategory, it says
Possible values are:
A - Airmail or Priority mail
B - S.A.L. mail / Non-priority mail
C - Surface mail / Non-priority mail
D - Priority mail sent by surface transportation (optional code)
But the 'Validation' for this field is
‘Should be one of the following values Priority/ Airmail’
What value shall be shown in this field?