1) Certainly. When a person buys Auctiva insurance, you don't have to do a thing (unless you have to make a claim). The package is automatically insured, and you print out labels from Paypal like you normally do.
You can also disable Auctiva insurance and purchase PO insurance like you usually do. But Auctiva's insurance system is so nice I recommend you at least give it a try. Also, you make a little money from it, I've earned almost $10 from using Auctiva insurance (It's actually U-pic insurance), and I'm just a small time seller.
2) You can do calculated shipping with Auctiva, too. Just get weigh your box, select Calculated Shipping instead of Flat Rate, and enter the weight. It uses the same shipping calculator the ebay does.