Models that come with piping will not incur this additional piping cost (unless it's contrast piping): these include the Manstad, Ektorp, Ekeskog, etc. Only models that originally do not have piping (such as the Karlstad, Hovas, etc) would incur the extra 10%.
Piping costs for cushion covers would be between $5-$15 extra depending on how much fabric is required :)