If you sometimes sell more than one item to a customer, you will probably not want to use checkout. Auctiva's checkout is not yet capable of combining multiple items into one invoice, so customers will have to pay separately for each item, or you will have to generate invoices from PayPal for these purchases.
As far as insurance, you can make that decision on a per-item basis. When you sell something and want to use Auctiva insurance, go to the Sales tab. There you will see a list of your recent sales, and there will be a link next to each item which will allow you to purchase insurance. You can also email a notice to your customers which shows them insurance information for their item.
I use Auctiva insurance whenever I can - it is cheaper than USPS insurance. If you charge the standard USPS rate, you will make a little bit of money. And every little bit helps!