Please contact Support directly with your requirements. However please note that the may be a delay in their reply due to Chinese New Year (10th - 22nd February 2018(GMT+8) ). You can check out the PCB order page too. https://jlcpcb.com/quote/pcbOrderFaq/PCB%20Thickness You should be OK to continue with your design on the assumption that a <=3mm PCB thickness is achievable as long as you are not also asking for things like 10 layers all with >2oz copper or inner metal layers.