Good question. It's been so long since I've had Windows 7 that I honestly don't remember, but I hate to say that I don't think there is a way to do it. You could set it to just check for updates, but then manually download them in the morning or something. That's about the only thing I can think of.
Your first month probably went well due to the initial 20 days of daily resets, so your monthly data amount really only had to last for ten or eleven days instead of the normal thirty or thirty one.
Hopefully you'll eventually be able to fine tune your usage so you won't have to buy anymore tokens in the future. It can be a trick, though, especially when you have multiple devices.
Well, they give the 20 days of resets to allow people to bring all of their devices to a current state without it affecting what would be a their normal monthly data allotment. Upgrades, updates, downloading new programs/apps, etc. Sometimes doing so can use quite a large amount of data.
Getting the devices set to NOT auto update is a big thing. Also, one of the other things is syncing. If you have Windows 10, make sure it's turned off, and make sure to turn off "Update from more than one place" in your update options. That one's really a pain for a capped service. It's under "Choose how updates are delivered", which is under the advanced options in your update settings. Using the cloud is also another bad one. Syncing and updating from more than one place can be constant data drains.
The actual updates for the OS's aren't all that huge, but "huge" is relative, of course. For my three computers the OS updates are usually in between one and one and a half gigs, combined. Then program/app updates are a bit more, but really not more than 2GB for everything, normally. Then again, it all depends on what you have, and what versions of software you have, of course. The difference in the sizes of the exact same updates for my laptop and desktop, which have the exact same OS, are ridiculous. The laptop updates were nearly twice the size of the desktop updates.