When the seller lists an item as 'Free Shipping' within the city of seller's operation, then the free shipping logo is displayed on the listing page. When the buyer from another city purchases it, the shipping costs are displayed.
In any case, the buyer is always made aware about the shipping price before they make their purchase.
The seller can't change the shipping costs after the item is sold. If the seller needs to charge more, then he/she would do it by adding the amount in the invoice.