I am a core developer for the Interchange project icdevgroup.org. We specialize in a Perl solution called Interchange6. This open source enterprise platform would allow you a scalable solution that would take from a few orders a week to full automated order fulfillment. You can get a number of open source PHP projects that will promise the world. But in the end your solution should be able to grow with you and adhere to your business work flow. My international team of developers are skilled in each area of the development process including SEO, multi-channel product sales (Ebay, Amazon,Google Products), software development. We specialize in using the Debian operating system and can host or help you manage an in house server. On the DB end of things we specialize in MySQL/MariaDB and Postgres.
To close we offer a fully personalized solution from ground up that is long term scalable. If this sounds interesting to you please let me know and I will write up a formal proposal.