You can't.
The seller cannot raise the amount of shipping from what she advertised. Nor can she change the price.
If you are willing to pay triple shipping (and I realize that it may have been an honest mistake on the part of a newbie), the only solution is a Paypal invoice.
I would suggest that she send you an invoice for the item plus the corrected shipping, referencing the eBay transaction number. When you pay, she marks the item as Paid manually on eBay.
If something sneaky was going on, which seems unlikely at the moment, then the single PP payment covers you under PPs Buyer Protection, which is nearly as strong as eBay's. If your PP account is backed by a credit card you are even safer.
If you were to send the extra money separately, which she could do as a Send Money request, then only the original eBay transaction with the lower shipping would be covered. If something bad happened you would not be able to recoup the extra $20.
Many crooked sellers do not realize that a PP transaction carries strong Buyer Protection. Let's not tell them. Let them find out on their own.
