To sell via the post you need three things.
First you need a mailing list
This can be researched by yourself or bought in from an outside supplier.
Second you need a warehouse and despatch system
There are significant discounts available with bulk postal mailings, first by using a Royal Mail account (rather than buying stamps or having a franking machine) and second by sorting the mail for Royal Mail and in return getting a sizeable discount.
You will also need a way of getting the items into envelopes - something that mailing fulfilment companies have.
Third you need a way of getting your post read
Most postal material is opened and glanced at, but most of it is then thrown away within moments. But if you write in a particular way you are much more likely to get your material read.
Finally there is a variant system for postal mailings to schools known as a “shared mailing”
The average shared mailing costs around 20% of the solo mailing price.