Strange, I have no problem using a Canadian credit card to purchase T-mobile airtime whenever I need some...I did find that some credit cards weren't accepted (Visa), but my Mastercard worked fine.

Interesting.... I've purchased T-Mobile USA airtime using my Canadian Visa while in Canada and never had any issues. If I remember correctly (the last time I did it was last Feb), the website gave me some grief about my postal code but i figured out a way around it.

Lots of online examples like T-mobile won't sell to me online even though I am a customer. I have to go through Walmart or someother to get time if in Canada.
Some Ebay sellers block my business from Canada even if I try to ship to a Washington receiver. If contacted they have sometimes agreed to the shipping and purchase.
I have encountered several retailers, I believe BestBuy.com won't deal with my Canadian credit, at least at one time they wouldn't.
If Speakout doesn't conform to Quebec specifc laws/requirements any sale in the province may be subject to penalties.

Strange, I have no problem using a Canadian credit card to purchase T-mobile airtime whenever I need some...I did find that some credit cards weren't accepted (Visa), but my Mastercard worked fine.