rsvp Help - Mail In and Online Wording

What we did was say
“Please reply by (date).
Let us know via phone call/text to (number), online at (rsvpify.com) or mail this card back in the attached envelope.”

We had an info card with our website and website password for rsvp and then also ordered a small amount of rsvp cards for mail in for our older guests in case they rather do that. But they also had the info card everyone else got with the website. I didn’t specifically say anything in their envelopes about doing one or the other. I figured it spoke for itself. No one ended up using the mail in and everyone did it online lol. So you can also do an info card with your website and rsvp mail in option and on the card say that you can rsvp via your website or by mail, whichever they prefer.

Pretty sure we just had "Please RSVP by [DATE]" on our invites - we had postcards and online options. Most guests chose one or the other but a few did both, though that didn't effect us at all - no one responded yes via postcard and no via the website, for example, and if they did, we'd just contact them to clarify!