Thievery is stealing. This is underpaying. While perhaps also unconscionable, they are two distinct activities.
The legal definition and moral definition are not necessarily the same, but the dictionary definition will suffice.
steal stēl/ verb
take (another person's property) without permission or legal right and without intending to return it.
If gas costs $3.40 a gallon and you pay $0.01, knowing that it is a mistake, you have just taken $3.39 worth of gas without the owner's permission. It's pretty cut and dried.