This fix won't work for everyone, but what we ended up doing was delete the variants we didn't want showing from our catalog at https://business.facebook.com/products/catalogs
For us our variants are mostly multipacks of the same product, so we wanted to make sure the lowest-priced products would show in our Facebook catalog/Shop rather than the more expensive 2 or 3 packs of the same item.
Since Facebook was haphazardly showing the 1 Pack / lowest priced items for some products but the multipack / higher priced items for other products, we went ahead and deleted the multipacks so that our catalog only shows the 1 Packs.
Hopefully this works and we don't run into other unforeseen issues. Will update this post if we run into issues.
OK I think i finally figured it out. It's a bitch to do but it looks like it's working for me. SOOOO go to product catalog in FB and click on any of the items that have ur expensive varient first.
1. Click expensive item
2. Click edit, then click on variables
3. You have to click on each variable that you don't want to show (so all the bigger sizes for me)
4. One at a time you click edit/then un-click product visibility so that the expensive ones don't show.
5. Your left w/ just the cheapest variable and it's good to go. Only thing is that I'm not sure how it effects things if your trying to sell through fb/insta. It was such a hassle for me to transfer my 400 products over that i went back to just redirecting people to my website to buy.
HOPE IT WORKS!