Actually you can group purchase orders. And there are modules that group POS orders. You can develop a module that does that

There is module for doing that :

https://www.odoo.com/apps/modules/8.0/account_invoice_merge/

I have attempted to make this work without success. In stock.py on line 1096 the action_invoice_create method has a parameter "group" and below the comment says it is the flag for a group invoice. So I put OpenERP in Developer Mode and created two Incoming Shipments from the same supplier and processed them both through to Received. Then in list view I selected the check boxes for both shipments and then chose Create Draft Invoices from the "More" menu. The dialog presented has the checkbox for Group by Partner which shows as boolean when moused-over in Developer mode. I'm assuming it is the parameter mentioned above. However after checking it and clicking Create I get an error -

IntegrityError: null value in column "account_id" violates not-null constraint.

I realize this may not be a very helpful answer but hopefully it will point you in the right direction so you might figure it out. At least you know now where you can create a group invoice....