ORS § 87.694
An owner may charge and collect a reasonable fee for an occupant's late payment of rent due for use of an individual storage space in a self-service storage facility. The rental agreement must specify the amount of the fee and the date on which the owner may charge the fee. The owner may charge the fee once in each month in which the occupant fails to pay the rent due or pays the rent due later than the due date specified in the rental agreement. For purposes of this section, a fee is reasonable if the fee does not exceed the greater of $20 or 20 percent of the monthly rent due for the individual storage space. The owner may charge and collect the late fee in addition to any other fees authorized by law or under the rental agreement.
ORS 87.694