As far as I know, in order for an e-stamp solution to work with Word
mailmerge, its developer has to develop some software to respond suitably to
the mailmerge events. There are one or two documents on this on the
Microsoft site. So in theory it's up to the developer/supplier to do that.
My guess is that they might need to work with Microsoft a bit to get it
right, but your best bet is still probably to contact the Royal Mail and ask
for the feature. If they've already developed that stuff, maybe they'll tell
you how to use it - I don't think it requires any intervention from
Microsoft to make it work.

Peter Jamieson