Some users, particularly temporary visitors in the country like exchange students, order a giffgaff SIM, then have to throw it away after a few weeks or months of use only (see this thread for example).
Same with users disappointed in giffgaff, who switch to a different provider quickly.
Now I don't know how long SIM cards are designed to be used for (five years?) but this sounds like waste.
Could we imagine a scheme where people can "give back" their SIM card to giffgaff when they don't need it anymore? So giffgaff can have them re-used and sent to new users, instead of creating new ones?
But maybe the admin involved may cost more in time and money than any saving? It reminds me of when I worked in an office and occasionally a customer would receive letters or documents from us in separate envelopes on the same day - this looked wasteful but it was not normally 'time efficient' for someone to identify such duplications.
Given the way in 'missions' giffgaff encourage members to order sims they neither want nor need, presumably giffgaff see them as having a rather low value - a throw away commodity. Never mind the recycling of previously activated SIMs, I wonder what percentage of SIMs distributed by giffgaff through the various channels are ever activated. Less than 50% at a guess.
I suspect the biggest costs of providing a sim are:
printing the mailer and
Unless you can devise a scheme where the sims are recycled on a person to person basis (without involving any postage) recycling would still require printing a mailer and involve two lots of postage. You also have the problem that in most cases they're no longer the 3-in-1 sims that are normally issued and you have to find a way of fixing the sims to the mailer, because there's no longer a sim carrier to fix to the mailer.
Recycling always seems to cost more than the value of the recycled item, but for larger items it is still essential that we recycle them because of the environmental impact if we don't. But sims are probably too small to justify spending extra to recycle them.