Yes, as long as there are enough funds available to authorize the purchase. Bear in mind, when paying at the pump, a hold may be placed for more than the actual transaction amount. It varies based on the purchase type (debit or credit) and the merchant. When making these types of purchases, you'll need enough funds in the Focus card account to cover these holds or the purchase will be declined.
You can avoid the discrepancy by paying the station attendant/cashier directly before pumping.
*In the event you don't use it all, the final transaction will post for the amount actually used, not for the amount authorized with the merchant. For example, you pay the cashier $40 inside. You end up pumping only $35 worth. The $40 authorization stays on your account until the charge is posted; however, it'll post for the $35, not the $40 authorized with the cashier.