There was a place where you could mark off that buyers could not fill in their own shipping cost but that option hasn't been available for a while as ebay stopped allowing buyers to make any changes on an invoice.
Usually, if there is no shipping charge listed then the buyer has to wait for the seller to send an invoice with the shipping charge. If the shipping charge is already in the listing, then the buyer can pay right away. So if buyers are able to pay the invoice that ebay sends out after the purchase then either the shipping price is already listed under the shipping tab or there is a glitch on the site that is allowing buyers to fill in a shipping cost.
What is the item number of a purchase where this happened and which country was the buyer from?