Why Does Everyone Want the Deposit Before They Show Up?
Money changes who is waiting on whom, and everyone in a trade understands that long before they master the trade. The deposit comes first because of: 1) the endowment effect, once your cash is in their account it becomes something they own and you have to ask for 2) loss aversion changing sides, you now have something real to lose by walking away and they do not 3) commitment and consistency, because a paid deposit makes you defend your choice instead of shopping it. A deposit for materials is a receipt. A deposit for nothing is leverage.
What is actually happening
A deposit moves the waiting
Your money becomes theirs to hold
Walking away now costs you
Tie it to materials, not goodwill
Myth vs Reality
Myth
A big deposit proves you are serious
Reality
It proves you will wait. Serious is a signed scope and a date.
Ask what the deposit buys. A real answer comes with a receipt.