That's a hard question to answer.  The quick 
answer is the line is different in every 
situation.  I have put in the description of my 
auctions that I ship to the US only.  It doesn't 
help.

Just this week I had to go to the post office and 
ship something to Singapore.  However, the 
bidder/buyer was polite in the emails and paid the 
increased shipping fee, so while I was grumbling 
(It's much easier to put postage on a box and let 
the mail carrier pick it up at work) I wasn't too 
serious about it.

What does bother me is the pressure I get from 
bidder/buyers AFTER an auction to 'upgrade' my 
PayPal account so they can pay with a credit card. 
Sorry, but no thank you!  I do not wish to assume 
a 30 cent + 3% charge on EVERY incoming payment 
(credit card or no) just so a bidder/buyer is 
inconvenienced less when he or she didn't read my 
ALL CAP warning about credit card payments first.

So there is my line, I suppose.  I'm not going to 
assume the responsibility for part of the payment, 
be it a fee from PayPal or increased shipping to 
sell an item to someone.  Hey, if I messed up and 
said that I would ship a 2 pound item for $2.00, 
that's my goof.  I'll kick myself, but I'll pay 
the difference.  But if I said payment was x 
amount for US shipping and the bidder/buyer gives 
me an address that isn't in the US, that is not my 
mistake or responsibility.

My turn to be sorry for a mini-rant.  It's been a 
long week and the effort it's taken to complete 
two, opening bid only transactions is amazing. 

I guess I draw another line at courtesy.  Honest 
mistakes are made, someone in a rush might not 
read all of a description.  If they are courteous 
and understand that they have taken something 
fairly simple and complicated it for me; but are 
still willing to work with me (ie - pay an 
increased shipping cost, or understand that 
shipping might be delayed until Saturday when I 
can physically go to the post office) I am more 
than willing to work with that person so that they 
get the item they want, bid on and paid for.

Just my take on the issue!  ^_^