Postage Due Elwood Indiana To London England Cover - 1901ish?

I came across this postage due cover and it jumped out to me due to the green 'highlighting'; this is new to me.

I assume that it is not possible to definitively say if this 'highlighting' was added after the fact or if it was done by a clerk wanting to really make sure the postage due did not go unnoticed...

Opinions?

Has anyone seen this before or perhaps have a 'highlighting' PD cover like it?***

Don
***There is a postcard thread here with similar PD marking.
https://goscf.com/t/68515

If it is contemporary, it might indicate the postage due was either waived or collected. I would not expect a clerk to highlight it.

Whitney Type #431 Handstrike 3d (Three Pence) Foreign Section
Not seen green highlighting before in 20 years.
Would suggest prev owner personal mutilation.
Albeit, unaware of any American routines.

Collect Brit. Pmks
Whitney
Page 221

What does the royal court of justice do?
Image result for royal court of justice london
Royal Courts of Justice, also called Law Courts, in London, complex of courtrooms, halls, and offices concerned primarily with civil (noncriminal) litigation.

I think NSK is right about it being waived.
A T tax marking has been marked over in red.(Considered)
from;
Philatelic Specialists Society of Canada
[PDF] UK Taxe Marks and their Offices of Use in the Centimes Era

Thank you very much for the insights, being waived appears to be the most likely explanation.
Don