I am not sure where i stand with this one.
The sums are pitifully small. If you ordered a gate from an online joiner, with the intention of fitting it yourself or using a chippy, you would expect to pay the online/supply costs upfront.
Personally, as a small fry decorator, I probably wouldn't provide a written quote for such as small sum. That said, in 95% of cases I have been recommended by a customer. If a stranger asked me to provide a price, I would probably give them a written quote and might set up a payment schedule. Many of my long standing customers just ask me to to do the job and pay me on a day rate.
My current job... I have I under quoted the exterior of a detached double fronted house by about a month of labour (three elevations). I was recommended to the customer by the guy pointing the brick work. The customer appreciates that I have come across issues that could not have been foreseen and has already said that he will pay me more than the initial quote.
One of his neighbours with an identical house has asked for a quote. I bumped the price up to £11k and added that any significant epoxy resin splicing on the sliding sash windows will incur additional costs. They have accepted the price and are currently working out their finances given that they also need to pay for an orangery and have one flank of the house re-pointed.
That said, my GF was ripped off when some checkatrade type A hole asked for over £3,000 to buy materials and just kept the money.
You clearly don't feel comfortable, so find someone else, which is easier said than done though.