I got some advice on this from both Andy and Christian, and the predominant message seemed to be don’t undercharge yourself. However, I’m really wary of them thinking whatever I ask for is far too much and as a result being less willing to ever ask me to do something again. However, this invoice will just be an estimate and when I spoke about it with Dan the manager of Neon Cactus, I think I gave the impression that I would be up for some form of negotiation if it seemed too much. I broke it down in to material costs, then added the cost for my time at the end. All I can do is send it to them and see what they say. I have sent a copy to Christian first and am waiting for his reply.