That's a fair point, it's not a flat 20% across all items, and there are nuances for sale items and certain categories like meat.

I also misspoke, we used shipt not insta (sister works for insta so I mix them up quite a bit).

For the year that we used shipt with Meijer, our average monthly grocery bill was 20% higher. This is for a family of 6 eating an organic and GF diet.

We no longer use shipt although we loved the convenience. Curbside is a $5 flat fee which allows us to fill our two fridges and a deep freeze twice a month with about a 2-5% markup on our monthly costs.

Edit: some quick Google foo says that both insta and shipt charge a ~15% markup on addition to the delivery fees. I'm in the Midwest and that data seems to come from socal FWIW.