Personally as it stands, if I'm daft and don't factor in my overheads, I'll eat the cost rather than upset a customer, this goes for any overheads, if I forget to include shipping in their final total I'll pay it myself and chalk it up to my own stupidity, the only exception is if I give them a total including shipping and they don't pay the full amount, that's a client mistake and I'll reject the payment and re-invoice in that case.

I think you're going to find it varies by artist, it is nice if you are kind enough to pay the fees but generally it should be assumed that they're factored in already as part of the overheads. (and tips are always welcome :D)

The artist has no right to with hold artwork since Paypal's terms of service make it violation to get the customer to pay the fees. If they're unhappy with the payment because you didn't pay fees (which they're not supposed to make you pay anyway, their price should cover their overheads, if it doesn't that's their problem) they really should either eat the fees themselves or refund you fully while cancelling the commission.