What Auctiva is asking you is how much will you charge someone for shipping for each additional item they buy if purchased at the same time.
In the Shipping section, you put in the shipping cost for the first item, then just to the right, fill in for each additional item purchased at the same time. This second shippng fee will only take place if the buyer purchases more than one at a time. So you can put the same cost as the first one or discount it a little or make it free. Your choice but you have to put something in that second box if you have more than one to sell.
I have free shipping on everything so it's simple.
I have found, for me, GTC works best. If I update something in an active listing, I don't have to remember to change it in Auctiva. And it's automatic; a big time saver.
Also, supposedly with eBay's new Best Match, the more you sell the higher your listing goes. That's why I list in multiple quantities with variations as much as possible. OOAK usually get buried unless you have a killer price. Also, with GTC, the number you have sold rolls over to the new listing. But if you creat a new listing, you start with zero sold.