On DH's Amex Gold, I stacked the 10% online grocery store up to $10, with Instacart 2500 MR for $50+ up to 2x purchase, and used a link for an extra $10 off Instacart.
$114 purchase from Costco, after all the stacking was a net $26 cost.
($10 AO, $10 link discount, 5000 MR x .0125 Schwab cash out value, 416 MR for groceries x 0.0125 Schwab cash out value)
Interestingly, I didn't have to enter my Costco membership number (even though there was a blank field for it) and I used an Amex for a Costco purchase.
The prices were about 20%-30% higher going thru Instacart vs. shopping in-store, so this is not something I would do without an incentive.
Now to see if Amex does any clawbacks...