How many items your guest list needs, spread across price bands so nobody's stuck choosing between overspending and turning up empty-handed.
The affordable items go in the first fortnight. Everyone who books late is left choosing between spending more than they have and arriving with nothing — and some choose the second and feel awful.
Couples who've lived together for years end up given a second toaster, because the list was built around what registries usually contain rather than what's actually missing.
Most platforms charge a percentage of each contribution plus processing, usually borne by you. It comes straight out of what people intended to give.
It's the one etiquette rule almost everyone agrees on, and it's easy to get wrong. The website, yes. The invitation itself, no.