Being self employed you are expected to supply the main materials required.
Though if your client is requiring a large amount of postage then please speak with them about providing you stamps/stationery so that you are not incurring high costs to support them.
There are online tools that can easily allow the client to cover postage costs such as Smart Stamp, which the Royal Mail have a version of that allows the client to pay for postage and you simply print off the stamps.
Also there are services where you scan in the documents and send them through online. The company then handles all the posting for you. The client again can cover the costs but you can simply handle the sending of the documents and liaising with the service.
Useful links below:
http://www.royalmail.com/corporate/services/smartstamp
https://www.imail.co.uk/landing_page/Default.aspx