Opening Mail on Shabbat

Question

I understand the similarity between opening mail and food products on shabbat, as the contents of both are needed in some way. However, in the case of mail, is there not a decree against reading "shitrei hedyotot," (documents, letters, greetings, bills, financial statements, etc) on shabbat? What are examples of contents in an envelope received on shabbat that are permissible to read? Thank you!