In Mailbox, function l2TransactionBaseCost is not called in the whole project, so it seems it will be called by the front-end to show users an estimation of their transaction cost. However, they do permit to pass an arbitrary _l2GasPerPubdataByteLimit whilst function requestL2Transaction requires that it is equal to REQUIRED_L2_GAS_PRICE_PER_PUBDATA, whose value is 800 (see here). I do recommend to hard-code the value in l2TransactionBaseCost to NOT return misleading estimations to users:
