Mailing cost (7)
I believe the answer is:
postage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mailing cost</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for postage that I've seen before include "It could be first-class" , "Cost of corresponding" , "Money charged for delivery" , "Amount paid to mail" , "Cost of sending letter" .)