I had one extra on my current car, cost £425 and the dealer asked me to cover the cost of the extra that I thought was fair enough as we can cancel these orders before delivery.
The price of that extra increased in price to £450 but only paid the £425 because that what was on the invoice at the time of ordering.
Both these things could be at the discretion of the dealer for instance in the case of price increases at the time of my order the manufacturer said in their T&C that the price was guaranteed at the order time price. Same goes for a deposit as a dealer may or may not asked for a deposit to cover the extras in case you cancel later and their under no obligation to order the extras so they could say no deposit, no order.
On my last car I was asked to pay a deposit of £500 to cover a £1K of extras but cancelled the ordered 5 months later due to slow delivery or late delivery times and the dealer stoned walled me for the return of my deposit for a car that I add, had not been built at that time. Had to get Motability involved to get the money back but don’t know if MB just thought it was a deposit on the AP or for extras, they probably didn’t know or care but got my deposit back a few days later.