Currently, we do not offer end-customer customization, however, it has been a requested feature and we are thinking of adding it to our product roadmap.
In the meantime, you can still accomplish your goal by editing products yourself before processing them.
For this, you will need to ensure that your order settings are set to manual, so that when a customers order comes in with the requested design adjustments, you can then create a product specifically for them and implement it as a manual order.
If you keep your order settings automatic, then you risk the customers order being sent to production without you making the specific adjustments.
If you have any questions, feel free to reach out to me or any other Printify Representative and we'll walk you through how to get this done.