$19.9+tax+15%代购费+shipping

so $22.6 already includes some shipping?

代购费 should NOT be included in the price...and it should NOT exceed 10% (if I rememberd correctly)

Plus, according to another building, monica lives in a place that does not have sales tax...If someone else gets it for her, why you are paying the 代购费???

So Miss Monica made profit already, lzmm, I don't think you owe her anything.