If you want to carry letters commercially, you must apply for a permit to do so.
You always need a licence for the commercial transport of letters if you collect, forward or deliver letter mail items. To obtain this licence, you must prove that you have the necessary reliability, ability and expertise.
If you are only carrying mail as an assistant for a licensed postal service provider, you do not need a license. You also do not need a licence if you are transporting letters that are enclosed with another item and relate exclusively to its contents (for example, an invoice).