To acclimatise ('acclimate' is what our american friend ssay

lol) the best thing to do is to very gently open the box when the cuttle arrives and let some light in... dont open the box quickly as it has sat in darkness for several hours and will ink if you open it suddenly.
Then untie the bag and over an hour drip water through an airline from the tank into the bag (much harder with an octo as they climb out!) then once an hour has passed and the water is fairly diluted with new water scoop the cuttle out and gently pour it into the new tank using a pint glass or similar. If you use a net you can damage their skin and it will most probably ink!
leave the lights off all day and dont feed it. let it settle in. Avoid all sudden movements and noises.. or you will get a blackout! If it is as big as 15cm it could easily fill a tank to the point you cant see in.
where are you getting it from?
The temp will be okay but that really is the maximum.
Is it being shipped in water with MGCl2 to calm it down? If so you dont want any of that water getting in your tankand its more tricky... find out..
Also, i dont have a cuttle tank at the mo, and the last one say about 20 or so up to 24 max i think
Cheers