This has to do with the way you set up your listing.
One case shows the retail rate for a 3 lb Priority package.
Four cases shows the retail rate for a 12 lb Priority package.
$21.80 is the retail rate to ship an 18 lb package to zone 2. Six items x 3 lb each = 18 lb. The system is doing what your setting instructed.
You have a shipping rule set up on this page that instructs the system to combine shipping by weight: https://www.ebay.com/ship/prf
Then that shipping rule is applied on the actual listing (or within the shipping policy if you use business policies).
You cannot ask the buyer for more $ - you essentially have 3 options here.
1. Ship and accept the loss for the listing error.
2. Cancel as out of stock and get a defect.
3. Probably the best option in your situation: evaluate your shipping method. You cannot downgrade from Priority to Ground, however you can look at the cost for a smaller box size (if safely possible) or even the cost to split up the shipment in 2 packages.
If the actual shipping cost is over $70 then it sounds like you're using an oversized box and getting hit with surcharges.
What are the box dimensions @ardvark_one ?
If you split into 2 different boxes that are both under a cu ft, at 9 lb each, the online discounted rate to zone to is $10.51 per box, or $21.02 total (probably less if buying the label on eBay).
