I would argue strongly that these two things are highly related.
Nobody attaches a physical price tag to their cake at the wedding - it looks immaculate, so you just assume it probably cost a lot.
Also likely why people get upset when it's NOT immaculate - it looks cheaper.
I imagine a fair amount of haggling goes on in cultures where that's a standard affair.
To overgeneralize a lot, but nevertheless I think that the trend is this way though it's nowhere close to an ironclad rule on any of these, petit bourgeois White Anglo-Saxon Protestants don't haggle over prices when planning a wedding. People of lower socioeconomic standing do, people of higher socioeconomic standing have agents that do it for them, and people of other ethnicities do because they don't have the cultural attitude that haggling is gauche.