1) Canadian wanted to buy a phone only available in the US, but was prohibited from using Canadian CC to buy it.
My inference is that the Canadian has an "Amerifriend" with a US address to which they want the phone shipped.
Some online sellers, especially of valuable merchandise like phones, will ship only to an address that's on file with the credit card. They do this as a fraud prevention measure. Likely the issue here is that the credit card has only a Canadian address associated with it. Most, if not all, credit card companies allow you to add secondary addresses, e.g. your work address so you can have stuff shipped to work. Some Canadian credit card companies, especially if they issue US$ credit cards, will allow US secondary addresses, e.g. because while the card holder may live in Canada they spend significant time in the US.
I bought a Google Nexus phone some months ago by first adding an Amerifriend's address as a secondary. The credit card was a Canadian TD/CT US$ Visa. I don't think it had to be a US$ card, but I have one and I wanted to pay in US$ in any case. The CSR at TD/CT Visa had no problem with adding a US address. After all lots of Canadian "snowbirds" fly south to FL, AZ, etc. during the winter so the request isn't unusual. I used the Amerifriend's address as both the billing and shipping address when I placed the order on Google's site. There were no problems with placing the order. The phone was shipped to my Amerifriend and my card was charged the correct amount. I picked up the phone in the US a couple of weeks later and declared it on the way back into Canada. (I was away for >48 hours so there was no duty or tax to pay in Canada.)
2) I was unable to buy Canadian iTunes gift cards with my US credit card.
3) The Canadians cannot use the US iTunes cards on their Canadian iTunes account.
Dunno anything about Apple iStuff. I'm an Android
4) A Canadian sent me a check which I could not deposit in my bank because it did not have a 9 digit routing number. He finally had to buy a money order and send that instead. I have not tried to send a check to Canada lately and have no idea if that could be deposited or not.
The format of MICR-encoded routing numbers differs between Canada and the US. This may cause the automated sorting systems to reject cross border checks and send them for manual processing. It's possible that some US banks don't want the hassle and so refuse to accept Canadian checks outright.
1. I've never experienced problems depositing US checks nor had problems reported to me by recipients of my checks.
2. I have a US$ checking account at a Canadian bank (TD/CT.) I always send US$ checks to the US. Even though the format of the MICR rounting information is Canadian, there's never been a problem. I always deposit incoming US checks to my US$ account.
As for Amazon, they generally don't have a problem with shipping books
overseas. I've bought books not only from .com and .ca but also from .uk and .de. In all cases I used my normal Canadian Amazon account. A few years ago I got an amazon.de gift certificate and had no trouble depositing it in my account. Keep in mind that Amazon charges outrageous fees for overseas shipping.
Now other parts of Amazon are another matter altogether.
1. As posted upthread, there's a lot of merchandise that .com won't send to Canada. I have it sent to my Amerifriend. Unlike Google (above) Amazon doesn't care if my CC billing address is in Canada.
2. OTOH yesterday Amazon was giving away some free Android apps
on their site. They wouldn't let me download them from Canada, not even using system apps that make it look like I'm in the US using a US carrier. The error message complained that the billing address in my account wasn't US. This is the same account that I routinely use with .com to buy merchandise to be shipped to the US.
So even within one company, policies vary among divisions. Go figure...