JLPT taken in Japan- overseas certificate delivery?

So I’m taking the JLPT in Japan this July, but moving back to the UK in August. The results certificates are issued after this, so I need to figure out how to get it delivered to my UK address. After piecing together various bits of information from various webpages on their site, it seems that I need to mail them an envelope big enough to hold the certificate, adequate stamps for JP->UK delivery, and a copy of my JLPT exam admission ticket. I would really like to know how exactly I formally make the request for them to do this for me (just randomly mailing them the supplies they’ll need out of the blue doesn’t exactly seem like proper procedure to me), and what kind of stamps I need for the JP->UK delivery. I’m unsure of the general cost/procedure of sending letters internationally here (air mail stamps?) and obviously I have no idea about the size and weight of the letter since I’m not the one sending it and just buying these supplies for them on their behalf.

Honestly I’m wondering if there’s a better way to do this. For lack of a better word this whole thing seems a bit dodgy to me. Would be nice if I could just make a formal request online and pay them however much needed, instead of being required to study up on the Japanese postage system and fetch them all these postage supplies like a dog. If anyone has experience with this, please let me know.