Only one credit can be used per order, but if you have more than one credit you'd like to spend then you can combine them into a single credit. Just make sure that you'll have time to spend it all - your new credit will expire on the earliest expiry date of those which you combine, and once combined we can't extend the expiry date further.
To combine your credits, sign in and head to My Credits, or select My itison > My Account > My Credits from the navigation bar.
First, select Combine.
Then tick each of the credits you wish to combine. You'll be shown the combined value and new expiry date before you save it.
If you're happy with the details of your new credit, just tap Combine credits to save your new, single credit which will be ready for use on your next order.
You can find out more about how to use itison credit with our handy guide.