Hi,
I looked at the listing and see that a buyer purchased qty 2, so I'm thinking there isn't a Combined situation/rules. It should have invoiced shipping by the calculated weight and method for qty 2 on the single line item.
Are you using eBay or Auctiva checkout?
Are you allowing the buyer to adjust the invoice, i.e did the buyer make this change?
BTW...rates to me in Columbus 43229 zip showing on the listing are $13.28 UPS Ground and $17.03 USPS Parcel Post.
Danno
Update: I looked at one of your other "Lot" listings with qty 2 available to see what it would provide for cost to me for qty 1 and 2.
Listing 270414475380 shows me the following for zip 43229:
Qty 1 / UPS Ground $19.20 / USPS PP $24.01
Qty 2 / UPS Ground $32.07 / USPS PP $37.99
It appears that this listing is doing the calc correctly, or at least is making an attempt (no free 2nd).
I would point to this listing with eBay Tech Support as reference of doing it right (at least from my point of view). It's obviously not doubling the number, as buyer (me as example) is assuming a single shipment/packaging. Does that make a different to the problem case?
Oh...and to repeat my result, just fill-in the qty # and zip down in the shipping section on the active listing to get a quote.