Is the seller in the USA or UK?
Is the seller enrolled in the Global Shipping Program?
Look for this on the listing:
Customs services and international tracking provided
If so, no she can't change or reduce or combine shipping on multiple orders. Another brilliant policy of the GSP.
Possibly more to the point, has the seller actually weighed and measured the curtains? Because having recently purchased a lot of drapes for my new house, I can testify that Blackout and Energy Saving curtains are very heavy due to the lining or treatments on the back.
I really doubt $12.94 would cover domestic shipping, and I'm pretty certain it wouldn't cover international shipping by a long shot. And that doesn't even consider the duty (on items valued over $20CDN -- about $15.50 USD) sales tax and the $9.95 Canada Post service charge.