It may vary by state, but I know that in Massachusetts, you can fill and purchase prescriptions at Costco without a valid membership.This is true for Dexcom supplies as well. BUT the special pricing talked about in that thread is a special rebate below their regular prices, and that special rebate is only for members, only for those without insurance, and is only for Dexcom G6. (I think the special pricing is for G6 only, and that is the only one I’ve asked about since that is the one I use. If you need the previous generations you should call and ask your local Costco pharmary.)
If you want to buy insulin or other diabetic supplies you just take your prescriptions to Costco (or have your doctor send them) and have them filled. When you go to pick them up, if they stop you at the door, simply tell them you are going to the pharmacy and they will let you pass. The pharmary prices are the same as those paid by members, and can be cheaper on some medications so it is worth including them in your list of pharmacies to call (if there is one close to you) when you need to fill a new prescription.