I've got:
-20gb on mega
-10gb on Google Drive

I have a cheap shell server that I would like to use to mirror this to *somewhere else* - probably a paid cloud provider, but maybe some distributed storage too.

Unfortunately the shell server only has 5gb local storage I can use for this.

How can I script something to do this?

I want something I can continue to use as sizes increase. A shell is great because it has high bandwidth allowance and is always on. (I don't have anywhere really put a NAS at the moment)
Ideally something I can later adapt to do the same for my wife's I cloud would be good.

or USB?

A usb isn't a bad idea. A 64Gb usb is less than renting space. Might fail.

Locally
I bought a 5Tb usb a few months ago for $120. They have pretty good soho raid nas for about $300. Qnap and synology and WD offer many programs to go along with those products. They are about the size of a small computer and run efficiently. You can put on top of cabinets and forget them.

Not sure why you need speed in a cloud backup but you can pay for almost any amount of room or speed on one of a few hundred sources. They might be backed up by tools like rsync or some ftp maybe.

A shell server is a virtualised environment... There is no physical computer to plug extra storage into...

Where can you get 30 Gig...Mega?

yes, mega gives 50gb free. it's works really well and there's commandline client. btsync is better if you have your own storage

You sound very informed on it.