You can use "Cash on Delivery" as an accepted offline payment method. Cash on delivery is enabled by default and available for all plug&paid accounts. You can add cash on delivery on an existing shopping cart or when creating a new one. 

Note: Cash on delivery is only available for physical products only. 

In your Plug's settings, under the Gateways tab, click Enable next to Cash On Delivery.  

The cash on delivery option will be available in the shopping cart to the customer.  

Once the customer completes the order, plug&paid will automatically generate an invoice with "Unpaid" status and "Amount Due" below the totals.

When you receive payment from the customer, visit your Invoices page and mark the sale as paid. 

plug&paid will generate a new invoice with an updated invoice status set to "Paid".