There is no tracking number and if the unhappy buyer goes to a Paypal claim,he will be refunded and if it is PP that does the refund, you will have a black mark on your selling record.
So.
Schmooze the customer. Tell her the date of shipping and the shipping invoice number of the Small Packet.
If it has been less than 30 days, try to get her to wait until then, but don't push it.
Refund the customer. Sob gently into your pillow. Ask her to refund the refund when the parcel arrives. Surprisingly, she probably will.
On Monday, phone Canada Post's customer service 1-800 line and explain what has happened.
They will give you a call back date. Mark it on your calendar and call then. The nice ladies can then cut you a refund cheque.
It's slow and annoying, but this keeps you off PP's radar and may even lead to positive feedback or at least no negative feedback.