I used this last week and it worked splendidly. Wish they were a little more transparent about the $35->free shipping offer being promotional, though. (After the first order it's $70->free shipping)
Could you please email cs@instacart.com with specifically what you mean (ideally, screenshots). We try to be very explicit about that but maybe we missed something. Thank you.
On that. You should be more explicit that CostCo delivery is $9.99 even for more than $70. I did not find anything about that on your site.