You should always ensure that all packaging is as robust as possible. Ensure your documents are packaged in a cardboard box that can withstand transportation within a large courier network.
How are my goods handled in transit?
Measure the dimensions for your envelope in the same way as you would a parcel, using 1cm for anything less than 1cm in size.
How do I measure my parcel?
When sending documents, you do not require a Commercial Invoice