There is PHP Warning occurred when trying to create invoice from template:
Warning: join() [function.join]: Invalid arguments passed in /home/smth/public_html/smth/wp-content/plugins/web-invoice/Functions.php on line 2290
web_invoice_payment_methods meta value, that plugin gets from DB, is stored as string. But plugin use join when trying to convert them to array:
$web_invoice_payment_methods = join(',', $_REQUEST['web_invoice_payment_methods']);
You are wrong. You need to use explode, not join :).