Royal Papers, Inc. (Royal) (defendant) had a collective-bargaining agreement with its employees, under which Royal agreed to contribute a certain amount to a pension plan each week. The pension plan was administered by Richard Diffley and other trustees (trustees) (plaintiffs). The trustees issued a memorandum to Royal and the other employers that were contributing to the pension plan. The memorandum stated that if an employer was late in submitting a monthly contribution to the pension plan, the employer would be assessed a late penalty of 10 percent of the contributions that were due for that month. Royal made two late contributions to the pension plan. The trustees sued Royal to collect a late fee of $210 for the two late contributions. The trial court entered summary judgment in favor of Royal, and the trustees appealed.