An online transaction can fail for a number of reasons - there could be a problem with the credit card used to pay the invoice, your online payment gateway settings, your merchant account, or anything else.  Here is a list of actions you could take when an online transaction fails in your FreshBooks account:

1. Confirm with your client:  Make sure that your client has entered their credit card correctly and their information is up-to-date.
2. Contact your online payment gateway:  Ensure that when you contact your payment gateway, you have the error message you received and know the transaction you are referring to.  More often than not, your payment gateway will be able to solve any transactional problems you are having.
3. Contact FreshBooks:  To report an online transaction failure to FreshBooks you can contact us with a screenshot/copy of the error message and the invoice number you are having trouble with.