Did you use Air Maill? If you did not, you may as well refund the customer now, with a polite request that when the item arrives that he repay the refund through the Send Money service of Paypal.
Surface Mail takes up to three months to arrive. Never ship overseas by surface.
What USPS service did you use? If you did not use Global Priority, which costs over $20 minimum, then you do not have Delivery Confirmation and cannot win a PP dispute. PP will not accept proof of shipping, they want proof of delivery. If you don't have proof of DELIVERY, refund your customer as above.
Twenty days is plenty of time for shipment from Canada (or the USA) to Germany if you used Air Mail.
You could ask very politely that the buyer wait until Day 30 for arrival, but if the item has not arrived by then, and you did not purchase a Delivery Confirmed service, refund.
Avoid negative feedback with prompt courteous replies, giving possiblemethods of dealing with the problem (ie : wait for Day 30), letting the customer know the invoice number and date of the shipping label/shipment, and making sure you are appearing aware of his unhappiness.
In other words, communicate. Most people are honest, and your customer is upset that the transaction has not gone as he imagined it would.
