Good Morning Julieo.
You're located in Canada so that means you'd be using Small or Light Packet and not First Class Mail. I'm pretty sure that was a typo.
I use Insurepost/Shipsurance (same company) and they do insure Light and Small Packet but not Letter Mail.
I use online shipping and when insuring shipments with no tracking I get a stamp at the PO when I ship to show the item was actually mailed.
I buy my labels on line so I use the transaction number for the label purchase in place of the tracking number on the form.
I also use the transaction number for the item itself for the transaction id they ask for.
I'm not sure what you'd plug in there when you buy postage at the PO but there must be a number on your receipt which would serve the purpose.
Perhaps for these items you'd consider buying your label on line?
You do have to get those items stamped as shipped at the PO so don't forget to do that.
I've filed several claims shipped that way and they've reimbursed me every one.