How do I stop a Priority International Flat Rate Envelop I mailed to Canada?

When delivering a Flat Rate Priority Envelope shipped to Canada, the wrong Declarations Customs Form was put into the clear plastic window. Now the wrong person will get this item. I called the USPS and they stated that there is nothing they can do, it shows it was scanned in the sorting facility on 10/12/2010 and they have no tracking until it is delivered to (wrong person). Can I contact customs or Canada Postal Service to recall my package?How do I stop a Priority International Flat Rate Envelop I mailed to Canada?
There is nothing USPS or Canada Post can do. There may not be anything Canada Customs can do (as it's them that will inspect it, if they do, not US Customs), but it won't hurt to call them.How do I stop a Priority International Flat Rate Envelop I mailed to Canada?
No you can not. All you can do is contact the wrong person and ask them to forward it.