JennBean and DtM, thank you so much for your responses!
I did check the Amazon website but they have downloads for the Windows version only, and I need the Mac version. But for a Windows user that may be a good option! As for Mac, Amazon will only mail the CD, as will Costco and Apple. I also tried my credit union (which is my credit card provider) and my brokerage, but they link to the Intuit Turbo Tax website where I ran into the problem inputting my billing address.
DtM, your message inspired me to call Intuit to see whether they could take my credit card details over the phone (thinking perhaps the form that staff use is different from the one I see online) and then email me a link and the serial/license code as you suggested. First, I had to speak with someone in the Philippines which was a little frustrating because I wasn't confident that she understood me and after about 15 minutes I was nearly ready to give up, but then she transferred me to someone in the USA and he had all of my details in front of him, based on what she had entered. He was super nice and after we chatted briefly he told me to log into my Intuit account (because I have used Turbo Tax in previous years, I have an account) and hey presto the software I wanted was there, ready to download. Which was rather confusing, because nobody had asked for my credit card details yet. So I was like, hey how did that get there? and he said he gave it to me for free, and was like, really, are you serious? thank you so much!! and he was laughing and so was I because I was ridiculously happy about this.
I don't know whether he gave me the software for free because I am a previous-year customer, or because he found me entertaining, or simply because it's the easiest solution for overseas customers with troublesome addresses.
Anyhow, the problem has been resolved!