votrechien - 6:11 pm on Dec 4, 2012 (gmt 0)
We had the same problem and I eventually had a custom system developed for the backend of OScommerce. It worked "ok" but it had the same problem as you- there was no way to prevent a user from double-processing an order (we used google docs and recorded the last order that was processed, which worked fine as long as everything was processed sequentially).
Your better bet is to look at a full on order management system. There's only a few of them...stone edge order mananger, dydacomp order manager, and Linnworks (the one we use). Linnworks does exactly as you've specified plus a lot more. They have a free option, but you're ultimately looking at around $60/month. It might be a lot for you to swallow if you're a small business, but it is really the best solution in the long run.