Hi Sebastian,
You can create a customer account explicitly (Webshop)
So in the customer account monitor in the receivables work center you can see a option 'New Charge/Credit'
You can two options here, 1. Customer Charge 2. Customer Credit and for these you can select a Revenue Account corresponding to the AR Account.
Select Customer Charge which will post as the transaction 'Other Receivables' in the customer account monitor for which you can apply 'clear manually' or pay manually by options.
This will act similar to your customer invoice.
Thanks
Lokesh Sharma