The first step in the calculation is to work out the Import Duty . I've no idea what the rate is on wallpaper but I'm confident that it will be well below 9%, so the amount calculated will be less than £9, which means that it will be waived anyway.
The next step is calculate the VAT. That's charged on both the price of the goods AND on the carriage costs. So, taking £20 as likely carriage costs, that's 20% of £120 = £14.40.
Then you need to add on the customs examination fee of £8.
So you're looking at a total of around £22.40 in charges.
Chris