eBay seemed to ‘guess’ the shipping on my items, I think some were relisted automatically with Simple Delivery and perhaps had carried the wrong dimensions - never completed this bit, no need previously.
I don’t tend to take the exact weight and dimensions etc during listing, as you don’t really know, until choosing a box etc. after sale.
2 sales recently were a PITA, the first had shipping as additional to the item price and I was unaware that eBay shipping was being used until sale. After waiting a day for a reply, eBay confirmed I would need to cancel the sale and relist with a new offer to buyer.
The 2nd, had to contact eBay again. I had always set this one to free postage. No need to cancel, the advisor said just use your own label if you would like as eBay refund any unused Simple Delivery labels if not used within 14 days.
So, perhaps set everything to free postage to be safer and adjust prices. There is no global setting to avoid the automatic eBay delivery being specified in the listing.