Is it permissible to combine the Dhuhr and Asr prayers, and the Maghrib and Isha prayers, due to the workplace being far from home and the large number of customers that prevents performing the prayer on time?

Prayer must be performed on time, and it is not permissible to delay it beyond its time, nor is it permissible to combine two prayers without a valid excuse that permits it. A large number of customers is not an excuse for combining prayers. One can close the shop or apologize to the customers to perform the prayer, as it does not take a long time. If one fears dismissal from work or harm to livelihood, there might be a concession for combining prayers.