In Switzerland every company sends invoices with bank payment slip. Normally is that an orange payment slip with reference number. In this number are serveral important informations. bank clearing, bank account no. and also your OpenERP invoice no. Question 1: Do you have implemented suisse orange payment slips in OpenERP yet?
Now you customer pays the invoice and you have an incoming payment on your bank account. From swiss banks you can download now your incoming payment-data as file. In this file you see the payment reference no., the amount and several other informations. With this data you can assign automatically which invoice is payed.
Question 2: Do you have this payment assignment implemented?