You can buy "a la carte" insurance policies direct from U-PIC. U-PIC is the same company that Auctiva uses to provide insurance services to their members.
Go to www.u-pic.com and choose "Buy Online". Be aware that when you buy directly from U-PIC, the restricted country list is larger than it would be if you signed up for "volume pricing", so make sure that you read the coverage rules, and note which rules apply to policy-holders and non-policy holders. The U-PIC customer service people are very helpful, so don't hesitate to call them if you have any questions.
You can, of course, sign up for volume pricing, if you insure enough packages per month to make it worth it.
Another option is to use a service like Endicia (postage label printing service), which also allows you to buy U-PIC insurance when you print out your labels. There's a monthly charge to use Endicia, of course, but it's well worth it if you do even a modest volume of shipments per month.