From Longman Business Dictionary
underchargeund·er·charge /ˌʌndəˈtʃɑːdʒˌʌndərˈtʃɑːrdʒ/ verb [intransitiveI, transitiveT] to charge too little or less than the correct amount of money for somethingThe city was grossly undercharging the tannery for the use of the sewage treatment plant.
undercharge somebody by £10/$20 etcThey undercharged me by about two dollars.
→ opposite overcharge —undercharge /ˈʌndətʃɑːdʒ-dərtʃɑːrdʒ/ noun [countableC]an undercharge of £15