I agree with the other posters, so how about learning from our mistakes? We have made them in the past.
Remove everything except your hand tools from the van, I mean everything, drills saws etc are extra or he hires them for you.
Customer will want a material list so add mapp gas, joint sealant, flax, screws, plugs, flux, solder etc to his list not from your van, remember if your van is empty you need all of the above with the order.
You won't lose everything on the job (maybe money) what you will gain is experience.
Another suggestion that if he doesn't think your price too low and gives you the job call into your local supplier, tell him you have a customer coming to buy a full heating system and ask that he holds your discount for you to collect in fittings, tube etc later.
Being brutally honest your day rate is way too low considering the level of responsibility you are taking on, a consumer is not the same as an employer who has a margin on the job, there is very little if any forgiveness from this type of employer.
Wishing you nothing but the best.