Update: I got a response from RS, and they refunded me back. They did mention that if I hovered over a little question mark by the delivery info, I would find out that they charge private individuals. However, in my opinion this is a poor implementation. First of all, they should have wrote this in their T&Cs (they didn't write it on the delivery bit but added an addendum near the bottom) and, at least refer to it on the delivery checkout page and not in a hover box (or make the totals add up properly, which was my main issue of contention). Although, I'm pleased with this conclusion, it doesn't explain why I got two different invoice amounts via email (not that Im too bothered by it now, just to mention it).