Shipmycards.com is an option but I think COMC is actually cheaper. Here's a comparison:
For 1 card:
COMC - $3 to package fee, $0.60 for the card, $5 flat rate shipping across the world, another $0.25 for shipping non-COMC card
TOTAL of $8.85
Shipmycards - $1 to receive, USPS postage for 1 card ($14.25 for rigid item), $3 for his trouble to package
TOTAL of $18.25
On top of this, you can combine the shipping with other COMC cards at no additional costs. I am not entirely sure how Shipmycards is competing from a price point of view if we are only talking cards. If you want to ship jerseys, shoes, bigger photos, then there may be some logic in using them.