Kickstarter only allow one pledge per backer.
This is really annoying and it's something that we've only just realised too.
For those wanting to add a roll bag to your existing pledge please go to your profile and select the 'manage my pledge' option.
Once there please increase your pledge by the cost of the roll bag you would like plus postage.
Postage will be in addition to any you've already paid for your original with a worldwide rate of £15 and a U.K rate of £8.