I find this whole thread redonkulous. HS drama isn't even this bad. I think splitting the cost would be the best for both parties, but either way its a bad ordeal. I'd be pissed too.
That being said, a couple "facts" caught my attention and I'd like some clarification on them.
#5...unless they changed something since I used paypal 6 months ago... The 3% is calculated when you send the money. I always paid the extra 3%, never had it deducted from the seller before. Maybe the amount could have been rigged, but usually you put in how much, and it just adds the 3% at the end and the sender pays. Could have been just me, but I call Shenanigans.
#6... After calculating the above, he still paid $110. He PAID for SHIPPING, so you may have lost money received but he still put out the $110 for them. If you had used a proper flat rate box, padded it properly, and stuffed it with both calipers and brackets you would have more in your pocket.