i have these and are great with one on i had 12 cans beer in and was fine easy hook on and off to with carry handle and the price great .. http://www.sjscycles.co.uk/sjsc-black-hook-on-rear-pannier-bags-prod11550/
I had ordered one of them from a uk shop they are rubbish i thought send mine back hope you like it because i didnt ,sorryI just bought this waterproof pannier bag from ebay for just under £15. It has a should strap which is ideal for me as I can also use it as a normal bag. It is being shipped from China so it should hopefully arrive by the end of this month. I have provided the link to this bag below:
15L Cycling Bicycle Bag Bike rear seat bag pannier waterproof free shipping | eBay