SAP Vendor Invoice Process

Hi Gurus,

I need to create a process to receive the payments from Vendors .Our vendors store their products in our warehouse so lets say if the products are not sold for 60 days than we send the invoice to our vendors for storage charges .What i need is how can we configure the process in SAP

1) What will be the PO/SO type

2)We are selling vendor product and the storage chanrges after some tie lets say 60 days will be chanrged from vendors

3) How often we can send invoice to vendors for storage chanrges? Monthy,Daily weekly ( Just an idea)

4)what are the config steps

Thanks in advance

Mike