We're running some e-commerce sites with 8-20k items through the $20/month plans and have never heard a complaint from Cloudflare. That said, any sites we 'care' about are running on their business or enterprise levels which are much higher than $20/month :P.
Probably you won't see this reply, but if you do... we move a decent amount, but not a crazy amount. In the last 30 days it was around 3TB total (through Cloudflare... we only saw about 2/3 of that).
