Cover of the day Boer War

[Ross Debenham]

My cover is a cover from Field Post Office 1 dated 11 September, 1900. I have ascertained that this post office was being used by forces engaged in action near the Drakensberg Mountains at the time. It is addressed to Cape Town. Obviously it has no stamps attached, but my question is why would it have been slapped with a 1d Postage Due fee when it arrived in Cape Town, where as it didn't seem to happen on mails to Britain.