Couples shower

You could register them for a cash fund, there are several sites that do that. However, being that showers are traditionally for gifts, some people might not listen

A shower is still for physical gifts, and even if you put cash in lieu of physical gifts, people will be put off on the wording, or still bring a physical gift regardless. It would be really weird (IMO) to receive an invitation to a shower with "cash only, in lieu of, etc" listed on the invitation, an dit would not fly in my circle.

They can register for cash funds (house fund, wishing well, honeyfund, etc) and post it on their website when guests go looking for a registry for wedding gifts. Alternative is stay away from the word "shower" and call it something else with no mention of a registry. Guests will most likely ask for a registry and you can refer them to the wedding website.

If they don't register, they will more than likely get cash gifts at their wedding. There's no need to throw a separate party to ask for more money.

That's super sweet of you! But usually people like to bring physical gifts to showers. That's why my husband and I didn't have a shower. And I honestly didn't care for one.

We did have a registry, but 98% of our guests just gifted us money lol.

That is really nice of you, but a shower is usually for physical gifts. There's really no polite way to ask for money at the shower.