When I really can't avoid a big download I use a download manager and collect it in chunks. This only works if the download can be resumed, which is usually but not always the case. There is also sometimes a time limit for the download, which means when you try to resume next morning you have to start again. Irritating, expensive, and sometimes impossible.
I have a 30MB/hr limit, 60MB/4hr, and 480MB/24hr, being on Ka Basic. I assume your client has a 120MB limit for the 4 hour period. This should allow the 180MB to be collected within 4 hours just about, since downloads are weighted to 50% of the size, while upload is weighted at 300%. For a large file the uploads are much smaller than the downloads, and 180MB will likely work out to cost about 120MB or less.
It does mean that the FAP will be operating hard at the end, possible stretching the download to the full 4 hours. Any other activity at the same time will eat into the allowance, and the 4 hour limit will be reached sooner, of course. That 'sliding window' really does operate as described. So split it and spread it out, or start it and go away for some hours. Just like dial-up, sadly.
Hope this helps.
John
Amendment:
I originally wrote that upload was counted at 150%. I don't know why I said that, it is in fact 300%. So 1MB upload costs as much as 6MB download! Of course, collecting that 6MB will require lots of requests to be uploaded, hence a 6MB download might cost 4 to 5MB.
Amendment 2:
Huh!
https://www.qtsltd.co.uk/Tooway.htm says that there is a 50% discount for night-time use. However, this is not mentioned by my particular outlet
https://www.bentley-walker.com/tooway/and anyway, I have KA, not KU. So the 180MB book should make it in 2 hours, not 4, and I'd better shut up, then.