As you have cancelled my order ,don't change anything .But if you can let me know why you cancelled it that would be nice .
What do you think of the use of past simple is it justified? I think so because for the second cancelled I don't mind if it has an effect on the present where as in the first one it has a present relevance, but may be it is better to use the same tense for both (past simple or present perfect) as it is the same action