Is there an organic preservative that I can add to homemade pesto to extend its refrigerated shelf-life?

I'm not totally certain whether you're trying to end up with room-temperature shelf-stable pesto. If so, you may have to look elsewhere. However, my family has grown bumper crops of basil before, and been left with the task of trying to preserve it for year-round pesto.

You noted that making pesto and freezing it doesn't work well due to the Parmesan cheese component. We would get around this by making a mix of the chopped/blended basil and olive oil, and freezing that combination. With that, you may also be able to add your salt & garlic as well, though I've not personally tried that, nor with including the pine nuts in the frozen package.

This does require the recipient to make additions on their end, but it does preserve the basil well. We would keep ziplocs of the basil-oil slurry in the freezer, and then done the end steps of adding the pine nuts/Parmesan/garlic and such.

I think that's fairly similar to what the answer in the question you linked does, but I can confirm that it preserves well and the mixture stays a dark green, not brown. If you're pretty sure that the packages are going to stay at freezer temperatures, I'd be confident in it working well. I'm not sure at what temperature you'd start having issues, as that's not something that we experimented with.

I typically do one of two things when I'm dealing with my end of year basil crop (trying to use it before that first frost) :

1. Freeze it: make pesto as normal, except for the cheese. Portion it out into some sort of small container, and freeze.
   I have an ice cube tray that I use specifically for this (as it would now make garlic-y ice cubes) and use 3 or 4 for a pound of dry pasta, but I've also used muffin tins. After freezing, move to a zip-top bag.
   The surfaces will discolor some, but it doesn't dramatically affect the flavor. If you're worried about presentation, you can place plastic wrap down in each muffin cup, drop in your pesto, then fold the plastic on top, doing what you can to minimize trapped air. I've also used large condiment cups if I'm making enough to give away (many Latin grocery stores will have an aisle with stuff for stocking food trucks; they're sold in sleeves of about 25, 50, or 100 depending on the store). You either fill them full to overflowing, or place a bit of plastic wrap or waxed paper down on the surface so it won't darken. When using the frozen pesto, you can thaw pucks in the microwave, but the ice-cube sized will typically thaw quickly by just putting them in a pot with the hot pasta. And then add your grated cheese.

2. I've also made basil infused oil, and kept it in the fridge (for those years when I didn't have enough garlic or nuts on hand, and I heard a forecast of a frost on my drive home after a long day). But the way that I've done it might be a botulism risk, so you might want to read up on the right way to do it.

I've been freezing pesto for almost two decades now. Although the basil oil is more versatile, I like the convenience of just throwing a frozen puck in the pasta with some cheese. (or thawing and slathering it on bread, etc)

I've never tried shipping frozen pesto, as I suspect that the shipping cost wouldn't be cost effective. If I were to do it, I would use 4oz condiment cups (or the equivalent metric size near 118mL; see above about Latin grocery stores), pressing some plastic wrap into the top before freezing. Once frozen, I would seal them in a zip top bag, and then ship it overnight in a styrofoam cooler marked 'live seafood'. (because a co-worker / former FedEx handler said they're extra careful with seafood ... because it stinks if it leaks or gets left behind).

Do what I've been doing for 10 years: freeze it in 4oz jelly jars, with 1/4" of headroom and a slick of oil on top. I have pesto that's been in the freezer for four years, and it's still quite good.

Sealing it in the jars, with the oil, minimizes oxidation-related discoloration. If you're really concerned about it darkening, then you can blanch the basil for 30s before pureeing it; this helps prevent later oxidation and keeps it a brighter green.

You do not need to leave out parmigiano; I'm not sure where you got that. Yes, freezing cheese does not do great things for its texture, but parmigiano should be a minority ingredient in pesto, and you are pureeing it with watery ingredients.

One possible reason you had a bad experience with frozen: you need to thaw it passively, not by heating it. Melting it over high heat will cause it to darken and the cheese may gum up.

Pinch of vitamin C

Disperse into salt 1st. Jar should have bubbles tapped/jostled out then covered in olive oil.

I get an extra week in fridge of not darkening.