I've bought stuff from the H*R store by the mail-in method
... you can get an international money order from the post office. They charge a fee... when I did it, it was $3.50, but the rate's probably gone up since then (it's been a few years - I stopped doing it this way when PayPal started supporting direct debit... but unfortunately PayPal is only an option for those 18 and over)... the website
doesn't say how much the fee is, so it'll be a surprise.
Then you just stick the order form and the money order (the person at the counter can tell you which part you send and which part you keep for your own records) into an envelope (I recommend buying one of those "International prepaid" envelopes - you can buy them individually, and you don't have to worry about figuring out the postage costs and buying stamps) and send it on its way.
According to this
, you'll need ID if you're sending more than $1000, but I doubt you're going to be buying that much stuff. But even so, they might insist on only selling to people over 18 (they don't say either way) so you may still need to get parents involved... and in that case, it might be easier to talk your parents into you just giving them the money and using their credit card. But then I don't know what your situation is like. But there's no harm in dropping in to the post office and asking them what's needed.