What is the ruling on one who delays Maghrib prayer until the time for 'Isha' prayer has begun due to traffic, and is that considered a shortcoming? Is it necessary for him to have the intention of combining the two prayers and to perform them consecutively, and what is the ruling on delaying congregational prayer due to transporting customers?

You sinned by intentionally delaying the Maghrib prayer until its time had expired. Your prayer, which you performed as a make-up, has absolved your responsibility, but repentance is obligatory upon you. What was incumbent upon you was to pray when there was a fear of the time expiring, or to invite those with you to pray, or to combine the two prayers if you intended to do so before the time of the first prayer expired. Your prayer is valid, and you must repent and not return to such an act. Congregational prayer is obligatory, but it is not required in the mosque according to most who deem it obligatory. Therefore, there is no harm in you transporting customers, but strive to pray in congregation as much as possible.