Is the cardboard box you are using suitable for posting? Do the contents of the parcel need to be secured in place? Answers to all questions relating to post-proof packaging can be found here - or downloaded as a PDF.
For business customer:
DHL's Package checking service for standard packaging products
Business customers can seek advice on the choice of their packaging materials and techniques for the use of standard packaging products and the shipping of hazardous goods from the experts at DHL's package checking service.
Please note that unfortunately the file is not available in English.