When ordering PCBs from JLCPCB I recommend:

Choose board thickness 1.2mm. It's easily thick enough at these sizes and is easier to cut.

Look down the page in the High-Spec options, where it says "Mark on PCB" and select "Order number (Specify position)". They will then position their number where it says JLCJLCJLCJLC on the PCB.