Often times, campaigns will notify their backers that if they add a certain dollar amount to their pledge, it will cover the purchase of a specific add-on or upgrade.
Once the backers are imported into CrowdOx, they will import into the reward level that they pledged in. If they pledged more than the reward amount, they will see that they have a credit to use on their extras page.
Here is an example of a backer that pledged $20 in a $10 reward level with $5 shipping. Once they get to their extra's page, they will see the $5 credit that they can use to purchase extras:
If the backer doesn't use the credit, they will see it at the end of their survey as a donation to the campaign: