Limio offers you the ability to integrate several payment methods, which include sending an invoice to customers. This is most useful when dealing with small businesses.
Adding 'Invoice' as a payment method in Limio
Campaign -> Offer -> Attribute -> Payment Types
Select your campaign then click on any offer to select Invoice as payment method. On an offer, select the dropdown menu 'Payment Type' and select 'Invoice'.
Downloading and Paying an Invoice