Six occasions, and the specification follows the tone rather than the other way around. Guests read stock weight and finish as a dress code before they read the wording, which is why getting the material right matters more here than almost anywhere else. Filter by occasion.

A suite (card, reply, details) several pieces must match exactly.

Heavy stock, restrained finish. Guests must read the formality correctly.

Flat, folded or shaped. The format should suit the party.

Distinct stock and finish. It has to survive the post-sorting glance.

Textured stock, minimal palette. Tone matters more than detail.

Fast turnaround, late changes. The date may move before it prints.
Match the event, not the brand. A relaxed company can and should send a formal invitation to a seated dinner, because guests use the card to work out what the evening is. Below about 250gsm a card reads as a flyer whatever is printed on it; from 300gsm it registers as an occasion. Textured and cotton stocks add a second signal (a surface that is visibly not office paper) which is why they persist for formal work despite the cost.
Order the envelope with the card, always. Matching paper whites afterwards is difficult and the mismatch is obvious, and a card that does not quite fit arrives creased. On a wedding suite, order the invitation, reply card and details card together so stock and finish stay consistent across the set, and allow more proofing time than a single card needs, because dates and names have to agree across every piece.