The material is cypress wood, approximately 10mm thick. It's slightly thinner to keep costs down. There are absolutely no issues with strength. Cypress is durable and can be used for many years. I also have another listing for a product made with a thicker board, closer to the actual thickness of a mail slot. Using that one will reduce errors caused by diagonal insertion. I've included a photo (the 8th one) to show the difference in board thickness. The photos are for size and shape reference. The item you receive will be of equivalent quality. This item was created out of necessity by myself, a Mercari user, who thought, "This would be nice to have." I am confident you will be satisfied with this product. Over 900 people have already purchased and enjoyed it. Cypress has a wonderful scent that is very relaxing. Wood is flexible and strong, becoming more polished with use, and is kind to nature. Above all, the texture of wood is superb. I've received many comments expressing surprise at how durable and pleasant to the touch the product is. I've been using mine for a long time as well. It doesn't bend flimsily like a thin ruler. It's sturdy, just like a real mail slot. You can check beforehand if an item can be crushed and pushed through. I don't believe there are any commercially available mail slot measurement tools with this level of thickness. If you would like a measurement slit of a different thickness (e.g., 1cm or 5cm), please contact me via the comments on the custom order page. It may take some time, but I will do my best to accommodate your request. I will use recycled materials for packaging. If you purchase multiple items and they can be shipped together, I will reduce the price by the equivalent of the shipping cost. Please note that I may change between YuYu Mercari便 and RakuRaku Mercari便 without prior notice. Thank you for your understanding. #TakaWataruHandmade #TakaWataruPackingScale #YuPacketPost4cm #YuPacketPost #ThicknessMeasuringRuler #ThicknessGaugeMeasuringRuler #ThicknessMeasuringItem #ThicknessMeasuringGoods YuPacket Post Thickness Gauge Measuring Ruler 4cm Thickness Measurement Ruler Scale YuPacket Plus Measuring Ruler Mercari Shipping Mercari Scale Shipping Ruler Thickness Measuring Ruler Thickness Measuring Item Thickness Measuring Goods Thickness Measurement Scale