Undercharge
/ˌʌndərˈtʃɑːrdʒ/
Definition 1 of 1
Pro Tip 1/3Mistakenly Low
Undercharge often implies an error or oversight rather than a deliberate discount. 
The cashier was new and undercharged me by mistake.
Pro Tip 2/3Perception of Value
Using undercharge can suggest the item or service is worth more, highlighting a loss or bargain. 
The handcrafted chair was undercharged; it was worth much more.
Pro Tip 3/3Discussion Trigger
Can lead to a conversation about the item's value or a correction of the price. 
I had to tell them they undercharged me for the meal, and we discussed its real cost.