You've translated correctly. But as I'm understanding, she's asking you to help with the cost if you desire.
This is what I got through translation software: "Hello, I sent you the scarf yesterday, the tracking number is on the site. I made a mistake concerning the price of the expedition which returned me to 28 € with the insurance ... I leave you judge if you want to complete, good weekend, cordially."
IMO, it's nervy to request though she's leaving it up to you to decide if you want to help. But as the seller, she should have properly calculated the cost.
I think I'd respond with something to the effect of:
"Dear seller,
I'm sorry it cost more for shipping than you calculated but I based my purchase on what my budget allowed and I can't afford to pay extra for shipping when it wasn't in the listing."
Additionally, ebay doesn't allow sellers to increase the shipping charge if the seller made an error but I'm guessing that the seller knows that, thus she shipped anyway and gave you the option of helping.