Look at the price table.

Look in the General Terms and Conditions to see what you cannot send in a letter. When sending objects, it's best to use a reinforced envelope or send it as a parcel.

Just like with electronic addresses, one letter can affect the proper delivery. Validate your address to be sure that it is known at bpost. The photo below shows how to write the address of the addressee correctly. Don't forget to mention the name of the addressee (required) and to put your own address on the back of your letter (recommended but not required).