Mixed Media - fixative question

I just sold a little mixed media work. It''s watercolour with dry carbon (lampblack) over the top. I'll try and attach a picture. The carbon is "loose" - i.e. it comes off on my fingers. It needs posting (to Wales), and I reckon I have two options: - mount it in some way so as to protect the surface and instruct the purchaser to take care - get hold of something to fix it with, then work out a suitable way of posting it. My question is, what would be the best fixative, if any? Secondary questions: - Are pastel fixatives watercolour-safe? - Am I right to dismiss my husband's suggestion of "spray varnish" (his example was branded Halfords).