I am wondering if the issue is the container for First Class ...

I get from the USPS shipping module:
Canada .7lbs
United States Postal Service (1 x 0.70lbs) (Express Mail International (3 - 5 business days)) $37.35
United States Postal Service (1 x 0.70lbs) (Express Mail International Flat Rate Boxes (3 - 5 business days)) $64.95
United States Postal Service (1 x 0.70lbs) (Express Mail International Flat Rate Envelope (3 - 5 business days)) $34.95
United States Postal Service (1 x 0.70lbs) (Express Mail International Legal Flat Rate Envelope (3 - 5 business days)) $34.95
United States Postal Service (1 x 0.70lbs) (Express Mail International Padded Flat Rate Envelope (3 - 5 business days)) $34.95
United States Postal Service (1 x 0.70lbs) (First-Class Mail International Large Envelope (Varies by destination)) $4.35
United States Postal Service (1 x 0.70lbs) (First-Class Package International Service (Varies by destination)) $9.15
United States Postal Service (1 x 0.70lbs) (Global Express Guaranteed (GXG) (1 - 3 business days)) $58.95
United States Postal Service (1 x 0.70lbs) (Global Express Guaranteed Non-Document Non-Rectangular (1 - 3 business days)) $58.95
United States Postal Service (1 x 0.70lbs) (Global Express Guaranteed Non-Document Rectangular (1 - 3 business days)) $58.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International (6 - 10 business days)) $28.25
United States Postal Service (1 x 0.70lbs) (Priority Mail International Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Gift Card Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Large Flat Rate Box (6 - 10 business days)) $53.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Legal Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Medium Flat Rate Box (6 - 10 business days)) $40.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Padded Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Small Flat Rate Box (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Small Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (Priority Mail International Window Flat Rate Envelope (6 - 10 business days)) $19.95
United States Postal Service (1 x 0.70lbs) (USPS GXG Envelopes (1 - 3 business days)) $58.95
Notice the Package is $9.15 and the Envelope is $4.35 ...

Going to the USPS site, I get the same quotes on Package and Envelope ...

Could this be the issue that you are experiencing?