as its a service i'm sure you could buy then pay on the day? even so sometimes you have to pay over the phone or online with some centres.
I'm thinking about having another 4D scan, i had one at 28 weeks and i want another i only saw some on ebay very briefly on a search the prices were good
i'm sure they would not be there if they were cowboys????
There are a few not very reputable scanning centres out there some are run by business people who only care about money, when i had my 4D i was brought back 2 extra times at no extra cost