Inventory stock order tracking system

Hi - what we are after is a basic debtor/order tracking software package. We sell a single product online and have retailers. The retailer call us up place and order (say for 10 units), we generate an invoice and then send out. We also offer a money back guarantee for returned devices, so they may out of the 10 return 2. At the end of the month we generate an invoice - in this case it would be for only 8 devices (as 2 were retured). We would like a program, could just be a fancy excel spreadsheet, that can generate the amount owning, show when orders were placed, show when items have been returned and credited for all our retailers and alos new retailers that come on board. The website for our product is www.ripsnore.com. we know there are a lot of fancy packages out there - but we are after just soemthing very very simple. Maybe even use a cloud based back end so it can be accessed anywhere.