Final answer:
Shawn most likely used a credit card to pay for the gas as he did not have enough money in his bank account to cover the purchase.
Step-by-step explanation:
A debit card is used to transfer money directly and immediately from the user's bank account to the seller. It is connected to the user's bank account and allows them to make purchases with money they have available. On the other hand, a credit card is a short term loan from the credit card company to the user. When a credit card purchase is made, the credit card company immediately transfers money from its checking account to the seller, and the user is billed at the end of the month. Until the credit card bill is paid, the user has effectively borrowed money from the credit card company.
In Shawn's case, he had $34.50 in his bank account, which was not enough to cover the $40 worth of gas. Therefore, since he paid for the gas and incurred a penalty fee, it is likely that he used a credit card to make the purchase.