When you click on the listing, then choose the Shipping and payments tab, - what does it say is the shipping fee to you? That's what you pay. A seller is not allowed to increase the stated shipping fee after the sale, - absolutely against ebay policy.
Or was there no shipping stated, then after the sale the seller sends you an invoice for a higher fee than she initially quoted to you privately? If that is the case, find the email from your ebay messages in which she quoted you the price, then from that message send her back a gently worded note explaining that you would like to have the shipping fee corrected to the earlier amount which she told you.
If you hear nothing at all from her, as you get close to that 'day 4', I'd say give ebay a call and tell them about it. If you are kind and polite to them, you'd be amazed what they can do, because ebay does not want a reputation for providing a haven for sneaks and cheats. In the top right corner of the page here, find the Customer Support link whereby you can get a phone number to call.
Even if you have to ultimately pay to avoid a strike, remember you have 60 days in which to leave carefully worded feedback. The more level-headed you can sound, the more likely you will be believed by future buyers.
Good luck.
