I can do this for 18% which ever you prefer since I have never traded with you before.
I am confused, so I pay $15 worth of bitcoin + 18% for your $15 PayPal?
No. 18% of going rate from whatever exchange you want to use. Just multiply that amount by 18% and add that on top of it. Member who offered you +5 on top of that rate(1:1) has the right idea

But he is taking a risk trading with no previous feedback and high chance of chargeback for only 5%

He is taking absolutely no risk in chargebacks.. I am wanting $15 PayPal I have bitcoin. I cant work with all of these %'s things really do not need to be confusing.
Just tell me how much bitcoin I need to pay you in order for you to send me $15 PayPal. I dont want percents, 1:1 or any of that. Just give me a figure.