ACH transactions can take up to five business days to resolve, depending on the banks involved.
When Fusebill integrates with a ACH gateways (EFT in Canada) the assumption is always that a successful transaction result from the gateway means payment was received. Fusebill marks the invoice as "Paid" , even though the transaction hasn't cleared through the banks.
You will need to monitor your gateway's payment status to determine if the transaction actually cleared the bank's process and has been finalized within the system. If a transaction is returned as "NSF" (non-sufficient funds), you should update the record in Fusebill to indicate this. Best practice is to process a cash refund against the invoice to reverse the effect of the "successful" transaction, Do not reverse the charges first. You can attempt to collect the funds again or write off the transaction if you determine that the funds are uncollectable.
See Managing NSF Transactions on ACH Payments for more information.