Sorry I can't advise on best way to ship back to Apple for a trade in. I think however that the following could be of interest to many if not yourself:
The main advantage for trade-ins is the convenience. Definitely NOT to get a reasonable price for your commodity. Whether it be a vehicle, a piece of furniture or an Apple product to sell, I would always first check national private ads to get a reasonable idea of the current value of your asset. Then take 3 good photos and post together with enticing description of (say) the iMac on a popular national site for personal ads. If the product doesn't sell within a month or so, it usually means the price is unreasonable, and lowering the price should attract more attention and an eventual sale.
As an example last year I was looking for (the last Intel) 2020 27" iMac, and had my 2012 27" model to sell. Had I traded it in for the new model it would have been practically worthless, but I sold it privately for 500euros (over $US500). Generally a win-win situation, especially if one has a little patience and time to spare.
In fact last year the 27" mid-range 2020 iMac was listed at close to 2500euros (US$2760) and there was "
no way" I was going to pay
that price. So knowing that a new 27" model was rumored this year (which we now know won't happen!), I looked around for an as-new model on the second-hand market and eventually found one for 1800€, made an offer for 1700euros (US$1900) which was accepted. Now I was never particularly good at maths at school but even by my modest calculations, by not buying new and not trading in my old iMac to Apple I saved myself approx a cool US$1400 - possibly peanuts for many but for others that relates to a 'free' new ipad or M1 MacBook Air. I always preferred economics to maths whilst at college......

Fwiw, this 2020 3,3GHz 512Gb SSD model iMac is a marvellous machine! Which in a year or two I plan to upgrade to a whopping 128Gb memory if & when necessary for my needs. Apart from reading about their technology, M1's don't interest me in the near/medium term until their reliability has been well proven. For those who must buy now, and plan to retain for greater than one year, imho AppleCare is a must!