I'm afraid I have very limited answers for you, since I don't use Auctiva insurance or checkout, but maybe I can be of some small assistance on your other questions...
First off, ALWAYS and without fail, buy delivery confirmation for every single package you ship. If you run into a dishonest buyer, they can file a claim stating they never recieved pkg and the burden of proof will be on you. Since you won't be able to show proof of delivery paypal will refund them at your expense. If you print your labels through paypal, Priority will give you FREE DC and 1st class mail is only .18. Add this into your shipping price on your next listings. You are also allowed to charge a handling fee (also added into S&H). Your time is worth something, not to mention packaging supplies (which you can get free at thr P.O. if you ship Priority).
*If you already know all this, I'm sorry, just want to cover all the bases
I believe Paypal only charges their fee on the final sale value of your item, NOT on whatever shipping/insurance you've included. Not positive on this tho, if I'm wrong I hope someone will correct me...I found a link with their fee schedule, I don't know if it will help or not:
http://www.paypal.com/cgi-bin/webscr?cmd=_display-fees-outside
And finally, as per your last paragraph, I myself only use Ebay's invoicing and checkout because I get alot of folks with multiple wins. (Auctiva didn't used to combine shipping that's why I didn't use them). So I don't know if this will even apply to you or not, but I have preferences set NOT to let the buyer edit total. Had too many of them changing things to what THEY wanted it to be
Hope I was able to help a little bit, and hopefully someone else will come along to answer your insurance questions. If not, I believe there's a tutorial on Auctivas home page?? It's here somewhere, I found their tutorials to be easily understandable.
Good luck to you!