Switch to cellular. Our iPhone plans have data caps too, but they offer a bit of a buffer. By switching off Wi-Fi on the phones (Settings > Wi-Fi) and using cellular data for most email and Web browsing, we trimmed our cable data usage a bit. Tethering an iPhone to a Mac using Settings > Personal Hotspot is another option, but would have quickly overwhelmed our cellular data plans.
Check Activity Monitor. If you suspect your Mac is using too much bandwidth but don't know which app is responsible, open Activity Monitor, click the Network tab, and click Sent Bytes or Rcvd Bytes to sort the list of apps by how much data you're sending or receiving. Turn off or change settings on apps that are overdoing it.
Having done all this and more, we still exceeded our monthly data limit. Thankfully, our ISP let it slide this time, but I'm now better prepared to be parsimonious with my data usage if the need arises in the future.
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