Petty Cash is a sum of money, usually kept at the employers place of work, used to reimburse employees for expenses they pay for out of their own pocket (eg: stamps, stationery etc.).
If you are self-employed (sole trader or owner/manager of Ltd Co.) you simply add these up each month and write yourself a cheque for that amount, as you would with normal expenses.
However, you will still need to keep receipts! Technically speaking you can not claim for something you have no proof of purchasing. Although I'm sure the odd book of stamps from a vending machine would not be questioned.