posting a cebeebies birthday card

[MissMooMoo]

Advice on the cebeebies website is to post it flat.
Card is A2 size, any idea on what to use to post it? Do the post office sell a posting bag that it will fit in?
Any advice from others who have done this appreciated

I have done this three times. I bought two pieces of card slightly larger than the birthday card and made them into a an envelope by putting the card between them and taping them together.

I got some A4 envelopes (the ones with card in them), opened them up and taped together. Both times I’ve done it the cards were read so they survived the journey!

Thanks both. Aiming to post off before Easter weekend otherwise I will definitely forget about it.
Card has lovingly been made by a 9 year old friend and I am hoping for her sake it is shown. She has done an amazing job (very artistic!)