I think the max is up to 4lbs - for regular/large non-FRE envelopes - to go First Class.
After that, regardless of the packaging, it has to go via Priority (and charged accordingly), so it's not worth it, really, except for the much lighter packages.
Here are the rates to Canada:
up to
1lb costs $4.98
2lbs costs ~ $9
3lbs costs ~$13
None of which, btw, explains 10 yards shipped for $10 :mrgreen: