Unfortunately, we cannot let the rider come back after they left the store. If you can still see or reach out to the rider without having to call him you can ask him to come back and take the missing product(s). The rider has a delivering schedule and is obligated to deliver the package within an hour.
What to do next:
Make sure that the rider already left, otherwise ask him to take the missing product(s).
If the rider left, you need to create another order. This will be a different rider that needs to pick up the order at the shops' location.
Make sure no other product(s) are missing, and you are shipping everything accordingly.
Send out a notification or call the customer to tell him that another order is on its way. The customer will receive another tracking code if the email address is correct.
The new rider that is picking up the order is also a new order on the invoice. You need to make sure that you always check the order before you ship it.