C5 reduced greetings card envelope
C6 white greetings card envelope
C5 is the ISO standard size for an A5 sheet (229x162mm), or an A4 sheet that is folded once. However C5 is rather too large to look snug around an A5 sheet, and we would suggest using this C5~reduced size, which is made especially for A5 cards. C5~reduced is sized at 216x151mm (A5 is 210x148.5).
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". You 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 bright white. 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 such items.
We also have a listing for more prestigious envelopes, for instance some using hand-made paper.