A reception-only invitation is for the guests who are welcome at the party and not at the ceremony — because the ceremony was private, or abroad, or fits thirty people. Four things make it land, and all four are wording.
Invite them to the reception, in those words. A reception in celebration of the marriage of on a formal card, to celebrate that evening on a plain one. Never at the marriage of, and never at our wedding — that is the sentence that has a guest dressed for a ceremony that is not there.
Say what has already happened, or what is happening privately. One line does it: are being married quietly on the morning of, or were married in April. It costs a line and it removes the entire misunderstanding. And if part of your list is not being asked to the party either — because there is no party, or not yet — those people are owed news rather than a card, which is elopement announcement wording.
Give a start time and an end time. A reception guest is planning an arrival, a babysitter and a taxi, none of which a ceremony hour helps with. 7:00pm until midnight answers all three.
Explain nothing and apologise for nothing. No due to limited numbers, no we wish we could have had everyone. A sentence explaining why someone is on this list is the only thing on the card that can make them feel second class; without it, the card reads as what it is — an invitation to a party. Whether to send one at all, and how the guest lists are drawn, is digital wedding invitation etiquette, which covers who gets invited to what.
Send both invitations the same week and on the same design. A reception-only card that arrives a month late, or in a plainer style than the one the ceremony guests got, says the thing you carefully did not write — when to send wedding invitations has the dates, including the reply deadline these cards share.