I assure you, I'm not here to grift business. Just trying to help answer your questions and provide sound advice. Mustang Market is a decades-long supplier of ours (NPD), and they're good folks, so I'm just trying to be honest while looking out for your best interests.
I went on the website, and they are using ACC's (Auto Custom Carpet) 100% nylon loop color codes, so I would deduce that they are offering ACC nylon carpet. If it were my car, I'd be opting for ACC Raylon loop. And bear with me here. Originally, your 1972 Mustang came with nylon loop carpet. 1967-earlier Mustangs came with 80/20 Nylon/Rayon carpet. So from an intuitive knee-jerk standpoint, one would be inclined to install the nylon. But here's the backstory. 20+ years ago the only factory in the U.S. that produced Rayon, closed up. And Rayon produced overseas was wildly more expensive. So Dorsett (the company that actually produces the raw carpeting) discontinued offering 80/20, and they developed a NEW loop carpeting that they called "Raylon". That word makes you think "Rayon", so you would be inclined to think "that's not correct for my '72". But here's the secret. Raylon is a 100% nylon carpet. BUT... It is very much improved over the old bottom-line Dorsett/ACC nylon. Raylon has twisted loops, and the loops are tufted in a random pattern, EXACTLY like every 65-73 carpet you'll ever see, be it 80/20 or 100% nylon. The ACC nylon offering (which is what Mustang Market is selling) does not have a twisted loop, and the loops are oriented in rows, aka "corn-rows".
So, bottom line, Raylon is a bit more expensive, but it's a 100% nylon thread, and it far-better resembles the original carpet that would have come in your '72.
Now, seat foam... It's not something that you want to price-shop. The quality of the foam is not just the shape, it's the actual chemistry of the foam and the characteristics of the foam. ACI utilizes a Dupont foam that is specially formulated for them, it's their own proprietary formula, and for almost 40 years now, it's simply the gold-standard. You'll do your seats once, and never have to do them again. Other foams out there, some are stiffer, some are softer, but none that I've ever seen pass the test of time.... They'll either pancake (like a cheap sofa caving under where your butt sits on it every day), or come apart underneath your upholstery. My Michigan store manager has been professionally installing upholstery for our customers for over 20 years, nights and weekends. He's used EVERY foam out there. And nothing comes close, nothing approaches the ACI. NPD isn't the only place that sells it, so if you can find it cheaper elsewhere, by all means... But this is one enthusiast to another, don't put new upholstery over cheap foam.
Sorry so long, I get going and have a hard time stopping LOL...