Cheapest rate: 'Printed Papers'
Package must only contain books, magazines, newspapers, leaflets or pamphlets. (You're also permitted to enclose a letter relating to the contents). To qualify, you must write 'PRINTED PAPERS' clearly in the top left hand corner.
Next cheapest (and best for your needs): 'Small Packet'.
Package may contain gifts, goods or commercial samples. (Again, you're permitted to enclose a letter relating to the contents). To qualify, you must write 'SMALL PACKET' clearly in the top left hand corner, and attach a CN22 customs declaration form.
Dearest: 'Letter'
Self-explanatory!
A sheet of A4 copy paper weighs 5g. So that's 30g for your letter. A pad of writing paper might weigh around 150g. A poster might weigh 60g. The cloth bag is hardest to estimate, without knowing the size or thickness, but I'll guess at 260g. Add 50g for the packaging and you're probably looking at around 550g. That's �11.34 as a letter but just �6.67 as a small packet:
http://www.royalmail.com/portal/rm/content1?ca tId=400036&mediaId=53800712
Chris