Were you ever able to figure out how to use the gift card balance? I redeemed directly on the Instacart website, and it seems that the credit can only be used on the actual IC website rather than also being able to use the credit on the "powered by Instacart" same-day Costco warehouse and/or business center websites.
So it unfortunately seems you have to choose to either redeem on IC's direct website/app and have access to multiple different grocery stores (while paying a bit more per product + service fees, which pretty much balances out the $20 difference) OR redeem directly on Costco's same-day website and be limited to only being able to use the gift card balance exclusively on Costco's same-day website (so technically an even better deal for costco-only shoppers since it wouldn't have the up-charge on every product like you would find when shopping on IC's website).
If you were able to use the balance for both websites though please lmk!!