Catching up on this from the weekend - I can't believe how badly that dress fitted!!
I am generous of boob (anything from an E to a GG depending on my weight at the time!) and I'll admit that I have in the past owned a few dresses with small cups a bit like her wedding dress. Usually bought because I was in a hurry, no time to try them on in the shop, just picked up my size expecting the cup to be more generous. It's then a choice between having the cup half over the top of your boob and your actual bra underneath a bit like she did, or having your boobs half out over the top, which is what I used to do and fine for a night out, but probably not the look she wanted for her wedding!
Thing is, picking the wrong dress for a night out is one thing - but for your wedding?! She had plenty of time and choice, cost clearly wasn't an issue (she could afford any of them, but obviously places would give her one for free so either way she could have what she wanted).
It must've been a sample size (as I think someone said previously) that they could let out at the back etc but not do anything for the cup size - and she left it too late/ was too tight to pay for one that was her size in the first place.
That said, when a friend of mine got married a few years ago, the shop she bought her dress from (and our bridesmaids dresses) were useless, so I have a lot less faith in bridal shop owners now! Despite as bridesmaids having to go and have our measurements taken and then fittings, my dress was too big (I did predict this when the shop owner wrote down she was ordering a 22, and I was a 14-16 at the time - when it arrived it was 3 sizes too big, they had to take loads out but it was still too big!), my fellow bridesmaids dress was too tight, all of them were too long, and the bride's dress was a corset, laced at the back, but there was no boning or an inbuilt bra or anything (although she was expecting there to be) and the lace up part was satin ribbons, which of course kept slipping. I retied it for her because it kept falling down 3x before we'd even started the photos! I think some places are just in it for the £££££, and unless you're really forceful they will just palm you off with something less than you're expecting.
However in Stacey's case I suspect her refusal to pay for anything is what bit her on the backside!