C5 greetings card envelope
C6 white greetings card envelope
C5, and C5 reduced, envelopes count as "Letters" with Royal Mail, but only if they contain a card. Anything much thicker (eg an embellishment on the card) will require postage for a "Large Letter". Your post office will advise on this, using a prepared slot through which the item must easily pass. Failure to pay the correct postage results in delay and a possible visit by the recipient to the nearest sorting office, and an extra fee to pay in addition to making-up the excess postage required.
We define Greetings Card Envelopes (GC envelopes) as those with diagonal gummed flaps and, normally, white paper of variable quality. Unless specified otherwise ours are of good quality paper, ie around 90gsm or more (thicker than everyday office paper), and of a good white colour. Some commercially produced, bulk-buy, cards are packed with envelopes of lesser quality. They are perfectly adequate for their once-use-then-dispose task. We have to limit our stock range, and have decided not to sell quality items, suitable for any purpose.
We also have a listing for more prestigious envelopes, for instance those made with hand-made paper.