We have a wordpress website which uses 'web invoice' plugin (version 2.1). There is a good chance that this plugin has been modified for this website so we cannot update it to the latest version.
The plugin only allows one invoice per customer. If a new invoice is created for that customer, the older one is removed.
We would like to modify it so that the invoices are not removed/deleted until specifically done so by the admin. This would mean multiple invoices per customer.