Spending other people's money. (work)...

Ok, chill.

If this is work, I assume this is what you do for a living? and at some point this person has decided that you DO know what you are talking about.
It's natural to feel a little uncertain when you get a big job, but If you can't trust your instincts, trust THEIRS, they obviously believe you capable, and 'supposed' doesn't come into it, compared to them you ARE the expert. be sure to include them in decisions, explain your reasoning for this and that, and don't think that you are solely responsible for what is really in the hands of mother nature herself, do your best, and let her do the rest.

It takes a while to adjust to a 'big money' mindset.

When I started out on my own, the first income cheque to the new company was for £845 inc. vat which just ab-so-lutely blew my brains - bloody hell, what a lot of money and I'm paying into my own company account!!

I was somewhat deflated that the bank wallah didn't congratuate me heartily (bottle of champagne etc etc).

I had to realise that the cheque I'd paid in (which was -at the time - the same as my monthly income) had oodles of costs against it, vat notwithstanding and it wasn't really all mine to spend on shoes.

Have confidence, Vicky - you are the expert in this situation - you might not feel brill, but you do know more than your paymasters - you will make better choices than they would, there will be more that plants that thrive than die.

That doesn't turn you into Alan Titchmarsh, but it does justify your 'expert' label and resultant fee.

Good luck with the project.