When you buy supplies on account instead of paying cash there and then your suppliers are your creditors until you pay them.
Because you will be using so much diesel you might decide to install a tank (for example 600 gallon capacity) so that you can take advantage of bulk discounts. If your coaches average 12 miles per gallon, 600 gallons would give you around 7,000 miles of travel. If you had three coaches, each travelling 100 miles every work day, you would need to fill up your tank every month.
The bulk fuel sector is very competitive and it's a good idea to shop around for best prices and terms. You will probably be allowed 30 days credit after the fuel has been delivered.
You might decide to have a diesel fuel card to use when you're away from the depot, either in the UK or abroad. The issuing company will want you to purchase a certain minimum amount of fuel every month, and will invoice you fortnightly or monthly for the fuel you have purchased. There's usually an annual charge for the card.