What do you mean by sellers insurance? Has the buyer opened a claim and now Paypal or Ebay wants you to send the tracking number for proof of delivery?
If you did use a service with tracking...Tracked Packet, Expedited Parcel or Xpresspost, then you should be able to see the tracking number on Paypal for that item when you click on details on the same line as the item.
Or, you should see it on ebay if you click on order details for that item.
It looks like you use small packet and light packet fairly often and neither of those have tracking and no insurance with Canada Post. The number that comes up on the label and that used to be automatically tranferred to ebay, is an invoice number...not a tracking number. If you used one of those services you can not prove delivery with paypal or ebay.