I've made several panellized pcbs with no extra cost in the last years.
Yep, sometimes they will ask for more money... When that happens, I cancel the order and try again, it'll eventually pass
However, I recently wanted to try some BGA devices.
Before doing the whole thing, I made some routing tests to ensure my design rules were up to spec.
I select "Pay after reviewing", so in theory the system should review the gerbers ensuring it mets their tolerances and capabilities.
Surprisingly (Or not, chinesium QA, huh), the system approves anything I throw at it.
I intentionally made ridiculous 1th thin traces and 0.1mm vias! And the system says everything's ok, ready for production! Wtf?
So they either re-check it after you pay, or they don't check anything at all, so good luck if you mess something up in the design rules.
Here's an example of what they approved! It's a BGA with 0.8mm pitch, 0.4mm pads.
So how are you really ensuring the DRC is met?