What should you do with the extra money you took from the customer for the abaya, which you did not tell the shop owner about, knowing that this is unlawful, and given your inability to inform the shop owner of the truth or reach the customer?

Selling at an inflated price is valid according to the majority of jurists, and the entire price belongs to the shop owner; you have no right to any part of it. You must repent to Allah by returning the right to its owner. The excess amount can be returned without directly informing the shop owner, using any means that do not lead to harm.