Build the piece around the job.
The design can be formal, minimal, personal or promotional depending on the relationship. The important part is leaving enough room for the message while keeping the brand present.
Cards can be coordinated with invitations, packaging, business cards and stationery so the follow-up feels like part of the same system.
Card size, stock, envelope needs, writing surface and quantity are quoted around how the cards will be used.
Project options
Use these as a starting point. The final specification is confirmed around the artwork, quantity, timing and intended use.