You can use this measurement tool to easily check your letters or packages if they exceed the maximum thickness for shipment. This measure uses metric units and common letter sizes (5mm, 10mm, 20mm, 50mm).
This tool is handy for when you dont have calipers, but even then they dont tend to reach to the middle of your packages so you cannot measure the thickness at the center easily.
I made two versions in case you need one with more reach.
Use a heated bed to prevent warping as this part is quite thin.
You can use a permanent marker to color the indicator numbers for better visibility.