I do a similar thing currently, but it's more to do with fees.  I add the cost of fees to the postage cost.  Whenever someone asks why my postage costs are higher, I'm honest about it.  I even asked eBay some time ago if I'm allowed to do this, and was told I can, but I just can't advertise it in my listings, which I feel is misleading, but thems the rules, I guess.  Very few people have an issue with this, and I do get repeat buyers, so I guess most buyers do seem to understand this.  Since eBay said I can do this via chat, I do have it in writing as well.