Credits can be applied in bulk - https://intercom.help/hauler-hero/en/articles/8180196-bulk-apply-account-credits

To apply credit towards a single customer / invoice -

From the customer's profile, go to Billing

Select Take Payment

In the pop up window, the system will display all unpaid invoices, the amount of credit on file and other payment options. To apply the full credit towards your invoices, leave the selected boxes check and select post.

Alternatively you can select to apply a partial amount or apply payment towards only one invoice, leaving any other invoice unpaid.

The same options are available under the Billing > Payment tab if processing bulk payments.