I highly recommend asking them to NOT glue on the top layer of foam (typically 1/2” blue foam) until after you have sat in it and done any necessary shaving of lumps and for lumps. When you try it out, bring some pins with you to temporarily hold the blue foam on while you get in it. Also… Unless you want to feel like you are velcroed to the seat… Bring a garbage bag or a silk shawl to lay over the foam. When you get it close to perfect, THEN they can start sewing the cover, and that will make it easier to hold everything together while you work on the final tweaks.
I also highly recommend ordering your own little foam shaver. They are less than $10, and make it soooo much easier to make smooth edits, just a millimeter or less per swipe, to the harder white foam. I’m not going to include a link because sometimes that delays the comment, but look for a “Stanley Surform Shaver”. I can’t imagine how many years of appointments I would have needed if I was not able to work on my new molded seat at home.
And before they make the seat cover, ask to feel like material options. Last time I did mine, I believe they had 2 types of material, and one was reversable (one side is more slippery than the other).
Good luck!!! I don’t need a new one right now, but I’m already dreading when I will. No matter how picky, annoying, frustrating, or demanding the process may make you feel, I promise I’m worse. Just take your time and keep working on it until it’s both comfortable and functional.