I paid $15.00 US and the download failed at approximately 300 megabytes, what do I do now?
I don't know what else I can contribute here -- it failed.
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As others have said, you don't have to pay to download Ubuntu -- the donation is optional. Therefore you can simply download Ubuntu again however you like. There's no need for you to donate again, just as there was no need for you to donate in the first place.
(I hope you weren't misled into thinking otherwise! It's great to donate, but everyone should be aware that Ubuntu is available officially and at no charge.)
Currently, the donation page gives you the opportunity to direct funds toward particular goals in Ubuntu development. So if you want to do that, that's one benefit of donating. However, donating doesn't enable you to download Ubuntu faster. Everyone should be able to download Ubuntu pretty fast, whether or not they donate (and no matter how much they donate, if they choose to do so), and everyone has all the same download options. So the second time you try to download Ubuntu will be no less likely to succeed than the first.
Sometimes your own Internet connection might have problems that interfere with reliable downloading. If your download is getting interrupted, is slow, or the file you get at the end is corrupted (broken), then as Peyto says you will likely be best off downloading Ubuntu with the BitTorrent protocol.
However, if you want to just try downloading Ubuntu again from your web browser, you can skip the donation page by clicking Not now, take me to the download! This is what the donation prompt currently looks like:
You can also, if you like, go directly to the main download server and use the links from there. For example, you can get Ubuntu 16.04.1 LTS here. You'll most likely want the 64-bit PC (AMD64) desktop image or the 32-bit PC (i386) desktop image. (Which one you should choose doesn't depend on whether your processor is made by Intel or AMD; the 64-bit release works on both 64-bit Intel and 64-bit AMD processors.)
Once you've downloaded Ubuntu, I suggest checking its MD5 hash to see if the file really downloaded successfully. This can save you some hassle -- it's better to find out your download was bad before you install than during installation.
If you're unsure what to do once you've successfully downloaded your Ubuntu ISO image, see:
You can download Ubuntu and not get charged any money.
Since I'm guessing that your Internet connection is slow I would personally recommend the torrent download. It is a bit better at not stopping or getting interrupted. Just go to https://www.ubuntu.com/download/desktop and click alternative downloads and torrents. Then go under the bit torrent section and download whatever you would like.
Or if you wouldn't like to deal with torrents, click Download instead and move all of the sliders to the far left so as to not donate any more. (Or you can click, "Not now, take me to the download.")