It could just be written like:

  (~> orders
    (filter (lambda (order)
              (timestamp> (order-date order)
                          (timestamp- (now) 7 :days))))
    (group-by #'order-customer-id)
    (mapcar (lambda (group)
              (reduce #'+ group :key #'order-total)))

But I prefer the typical Lisp code where I get the sums of the totals of the orders with the same customer ID which were placed in the past week, instead of the orders made the past week grouped by customer ID their totals summed together.