Adding values in one column to values of another column in a different table, SQLite

I have a customer table and an orders table. The orders table contains rows of orders each to a given id of my customers table. It is possible to have more than one row of orders per customer. At the end of the week I want to be able to iterate through my orders table adding the order total to the total amount that a customer owes. After reading that SQLite doesn't allow join and update to be used together can somebody point me in the right direction. Cheers.