Kiriska: As with many printers, the bulk of Catprint’s business is not in art prints, but in stationary, invitations, and cards. You can see this in almost all of their marketing. 

Usually, invitations and cards go in envelopes. So there are envelope options. Envelopes are not required. If you want them, the “right” one is whatever your preferences are. Is your holiday card red? Maybe a red envelope would work. Is your wedding invitation white with gold trim? Maybe a gold envelope. You do you.