This is a shitty practice and I'll be sad if you get a lot of support for it on this site.  This is a way of obscuring your fees and inflating your invoicing without the clients necessarily catching on.  You should charge what you're charged.  Raise your prices if you need to raise your prices.  Can you come up with any good reason to justify shifting your fees in the form of fake "copy costs" and not a straightforward flat or per-page additional charge?  Definitely not.  This is ONLY a way of disguising your fees.

Sheisty, sheisty, sheisty!