To add an "add-on" to your pledge you increase the value of your pledge amount.
e.g. You have pledged for "Home Alone" which was £35.00.
You want to add a Samsara vehicle (£30.00) as an add-on, so you change your pledge value to £65.00.
When the Survey is sent out after the KS ends our records will show that you paid £65.00 for a £35.00 "Home Alone!" pledge, so in the survey you tell us that the other £30.00 was for the Samsara.
To include multiple add-ons you simply increase the value of your pledge to include the cost of all the add-ons that you want.