I use Ruten all the time, but I wouldn’t use that seller. It looks like they don’t have the item in stock, but resell from Amazon. So if there’s no problem, you can just order it directly.
It’s doable from Amazon. Just choose a decent shipping service. There are other places besides Amazon to buy from. But if you really want that one, use service at the link you posted. That will work as good as anything else. They don’t ship to 7-11, and neither will Amazon, but they do offer COD. I have a hunch, though, since they are basically dropshipping, that they will contact you and tell you they don’t in fact accept COD and you have to pay in advance.
It’s assuming you are a Taiwanese citizen. Probably won’t work, but did you try your passport? You do need to provide your ID info for a large purchase, but you can also give it to the shipping company and they will deal with it. I’ve had to give mine to DHL many times. You may need to contact customer support to deal with that. I’ve also ordered from them, but not for a few years.