Hi There,
As per your project description you are looking for a magento extension using which you can allow your users to customize their product they are purchasing form your store. For an example a customer is willing to purchase a mug from your online store, once he is done with selection of product he can now choose to customize the mug, like print some image or add some text. I do have experience in handling this requirement. I have done similar code earlier, hence I can do this.
Looking forward to work with you!